Track .envrc + make postgres scripted test work on NixOS

[emsosams]

Summary

• Track .envrc (with use flake) so contributors with direnv + nix-direnv get the devshell auto-loaded on cd, rather than authoring the file themselves or running nix develop manually.
• Fix nomad/clean-and-migrate-postgres scripted test on NixOS. zonky's bundled generic-Linux Postgres binaries fail to load there ("NixOS cannot run dynamically linked executables intended for generic linux environments"). The devshell now exports NOMAD_PG_TARBALL pointing at an XZ tar of pkgs.postgresql (bin, lib, share); the test feeds that through a custom PgBinaryResolver and overrides unix_socket_directories to $TMPDIR (nix postgres defaults to /run/postgresql, which doesn't exist in the sandbox).
• CI on Ubuntu is unaffected: env var unset → EmbeddedPostgres.start() runs as before.

Closes #4.

Test plan

• sbt clean scripted doc passes locally on NixOS (22/22 scripted tests + doc).
• CI passes on Ubuntu (no env var set, default zonky path).

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Tracks a repo-level .envrc to auto-load the Nix flake devshell via direnv, and adjusts the clean-and-migrate-postgres sbt scripted test to optionally use a Nix-provided Postgres tarball on NixOS (avoiding zonky’s bundled generic Linux binaries).

Changes:

• Add tracked .envrc (use flake) and stop ignoring it in .gitignore.
• Extend flake.nix devshell to build/export NOMAD_PG_TARBALL containing a tar.xz of pkgs.postgresql in the layout expected by zonky.
• Update the scripted test Main.scala to use NOMAD_PG_TARBALL when set and override unix_socket_directories to the JVM temp dir.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
sbt-plugin/src/sbt-test/nomad/clean-and-migrate-postgres/src/main/scala/Main.scala	Adds env-var-controlled Postgres binary override + socket dir override for NixOS compatibility.
flake.nix	Builds a Postgres tarball and exports NOMAD_PG_TARBALL in the devshell.
.gitignore	Stops ignoring .envrc so it can be tracked.
.envrc	Enables direnv+nix-direnv to auto-load the flake devshell.

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[NovaMage]

Jhin🤖 says: Review finding:

This PR makes the repo flake-driven for everyone by tracking .envrc with use flake, but flake.nix still only declares Linux dev shells (x86_64-linux and aarch64-linux). Since we also have macOS contributors, this is incomplete for main: any Darwin developer using the shared direnv/flake workflow will hit a repo entrypoint that the flake does not currently satisfy.

I validated the NixOS-specific Postgres scripted test change itself and it looks sound; the blocker is portability of the newly shared flake setup. I would either add Darwin outputs in this PR or avoid making .envrc repo-owned until that support is in place.

[NovaMage]

Sylas🤖 says: Valid finding — confirmed with @NovaMage that macOS contributors do use this repo via nix-darwin. Fixed in b7f09fe by adding x86_64-darwin and aarch64-darwin to supportedSystems in flake.nix.

The NOMAD_PG_TARBALL override path is platform-agnostic (zonky's setPgBinaryResolver plus unix_socket_directories pointed at the JVM temp dir), so the same Nix-built tarball mechanism works on Darwin too — harmless there since zonky's bundled macOS binaries would also load natively. Verified the Darwin attrs evaluate cleanly from this Linux host (nix eval .#devShells.{x86_64,aarch64}-darwin.default.shellHook --impure --raw).