ci: revert test-elixir cache path fix

[prql-bot]

Problem

After #5826 merged at 08:44 UTC and ran on main (cache miss → fresh build → cache saved), the next run on main (the 11:04 UTC nightly, run 24955057135) hit the cache and failed:

** (UndefinedFunctionError) function PRQL.Native.compile/2 is undefined (module PRQL.Native is not available)
Failed to load NIF library: '.../prqlc/bindings/elixir/_build/test/lib/prql/priv/native/prql.so: cannot open shared object file: No such file or directory

Root cause

The _build cache from #5826 doesn't include the Rustler-built NIF. By default Mix symlinks _build/<env>/lib/<app>/priv to priv/ in the project root, and priv/ sits outside the cached path. The saved cache is only ~288KB (beam files only — a real NIF would be multi-MB).

On a cache hit:

1. _build/ is restored (beam files + dangling priv symlink)
2. priv/native/prql.so is gone
3. mix compile sees beam files as up-to-date → skips Rustler → no rebuild
4. mix test tries to load the NIF → ENOENT

The deps cache is fine on its own, but #5826 added them as a pair and the _build half is the broken one.

Solution

Revert #5826. This restores the prior state where the cache silently never restored — slow but correct. A proper re-add needs:

• Either include prqlc/bindings/elixir/priv in the cache paths, and make the cache key sensitive to Rust source (native/**, the prqlc crate sources) so a stale prql.so doesn't get reused when Rust changes;
• Or drop just the _build cache and keep the deps cache, accepting a fresh ~1.5 min Rust rebuild every run.

Leaving that design call to a maintainer.

Testing

The verification is the next nightly (or any tests run on main after this lands) showing test-elixir green again — same posture as pre-#5826, since this restores the exact prior workflow.

---

Closes #5830 — automated triage

[prql-bot]

A separate ci-fix session landed on the same root cause but went forward instead of back. Posting the diff here so a maintainer can pick between the two — happy with whichever lands.

The forward fix is also small: cache priv/ alongside _build, bump the cache name to invalidate the stale entry from #5826, and add Cargo.lock to the key so Rust dep changes invalidate the build cache.

--- a/.github/workflows/test-elixir.yaml
+++ b/.github/workflows/test-elixir.yaml
@@ -68,19 +68,29 @@ jobs:
       # Step: Define how to cache the `_build` directory. After the first run,
       # this speeds up tests runs a lot. This includes not re-compiling our
       # project's downloaded deps every run.
+      #
+      # Also cache `priv/`: Rustler builds the NIF via cargo into the workspace
+      # `target/` and then copies it to `priv/native/prql.so`. `_build` only
+      # holds Elixir artifacts (and a symlink to `priv`), so without caching
+      # `priv` the restored cache is missing the .so but mix still considers
+      # the project compiled and skips the rebuild.
+      #
+      # Cargo.lock is in the key because the .so depends on Rust deps; an
+      # update there should invalidate the build cache.
       - name: Cache compiled build
         id: cache-build
         uses: actions/cache@v4
         env:
-          cache-name: cache-compiled-build
+          cache-name: cache-compiled-build-v2
         with:
-          path: prqlc/bindings/elixir/_build
+          path: |
+            prqlc/bindings/elixir/_build
+            prqlc/bindings/elixir/priv
           key:
             ${{ runner.os }}-mix-${{ env.cache-name }}-${{
-            hashFiles('**/mix.lock') }}
+            hashFiles('**/mix.lock', '**/Cargo.lock') }}
           restore-keys: |
             ${{ runner.os }}-mix-${{ env.cache-name }}-
-            ${{ runner.os }}-mix-

Trade-off vs. the revert here: the revert is a one-line round-trip that gets CI green tonight and leaves caching for later. The forward fix keeps caching working — the elixir job currently spends ~2 min on the Rust build, so reverting concedes that on every run, including for the deps cache half (which was always correct on its own; #5826 just bundled it with the broken _build half).

Source for the priv/native location: the successful PR run for #5826 logged Copying /home/runner/work/prql/prql/target/release/libprql.so to priv/native/prql.so (job 73060377548) — the priv/ is relative to the job's working-directory prqlc/bindings/elixir/, not under _build/, which is why _build-only caching loses it. Also dropped the loose ${{ runner.os }}-mix- restore-key as a small drive-by — it would cross-restore the deps cache into the build slot.

[max-sixty]

do a full fix and link from here please

[prql-bot]

Forward fix opened as #5832 — caches priv/ alongside _build, bumps the cache name, and includes Cargo.lock + prqlc/**/*.rs in the build cache key so the .so is rebuilt whenever any Rust code that produces it changes. Drops restore-keys on the build cache to avoid partial-prefix restores delivering a stale .so. Description there has full reasoning.