ci: correct test-elixir cache paths

[prql-bot]

The actions/cache paths in test-elixir.yaml point at directories that don't exist in this repo, so the cache silently never restores or saves. The Elixir job redownloads and recompiles dependencies from scratch on every run.

Evidence

The latest tests run on main (run 24934608816, job 73017930250) shows both cache steps emitting:

Cache not found for input keys: Linux-mix-cache-elixir-deps-...
Cache not found for input keys: Linux-mix-cache-compiled-build-...

…and at job end:

[warning]Path Validation Error: Path(s) specified in the action for caching do(es) not exist, hence no cache is being saved.

actions/cache paths are repo-root relative, not relative to the job's working-directory. The Elixir binding lives at prqlc/bindings/elixir/, so deps and _build resolve to prqlc/bindings/elixir/deps and prqlc/bindings/elixir/_build, not elixir/deps and elixir/_build.

Origin

The mismatch was introduced in #3683 (the directory reorg from bindings/prql-elixir → prqlc/bindings/elixir): the working-directory was updated, but the cache path was only partially renamed (prql-elixir/deps → elixir/deps). The path was wrong before that commit too — prql-elixir/deps was missing the bindings/ prefix — so the cache has likely never worked.

Verification

This is a CI workflow change rather than testable code. After merge, a tests run on main should show:

• No Path Validation Error warning at the end of the Elixir job.
• On a second run, the cache steps logging a hit instead of Cache not found, and mix deps.get / mix compile skipping the work.