fix(ci): build each Burrito target on its matching OS runner (CRY-40)

[bougyman]

Summary

• Fixes lcls --full crash caused by MDEx's rustler_precompiled NIF being unavailable in macOS/Windows Burrito releases
• Root cause: the previous workflow built all targets on ubuntu-latest, so the Linux .so was bundled instead of the Darwin .dylib or Windows .dll
• Solution: each target now runs on its matching OS runner — macos-latest for macos_aarch64, ubuntu-latest for linux_x86_64 (unchanged), windows-latest for windows_x86_64
• Also removes the p7zip install step, which was only needed when cross-compiling Windows from Linux; Windows runners have 7z pre-installed

Changes

• .github/workflows/main.yaml: convert flat target matrix + runs-on: ubuntu-latest to an include matrix with per-target runner fields; remove the apt-get install p7zip-full step

Test plan

• Trigger a release workflow (or workflow_dispatch) and confirm all three matrix legs succeed on their respective runner types
• Confirm the macos_aarch64 binary loads MDEx correctly (lcls --full no longer crashes on Darwin arm64)
• Confirm linux_x86_64 and windows_x86_64 builds are unaffected

Closes CRY-40.

🤖 Generated with Claude Code

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[bougyman]

Implementation complete

This PR implements the fix for CRY-40.

What changed: .github/workflows/main.yaml — burrito-build job only.

• runs-on: ubuntu-latest → runs-on: ${{ matrix.runner }}
• Flat target matrix → include matrix with per-target runner fields:
  • macos_aarch64 → macos-latest
  • linux_x86_64 → ubuntu-latest (unchanged)
  • windows_x86_64 → windows-latest
• Removed the apt-get install p7zip-full step (was only needed when cross-compiling Windows from Linux; Windows runners have 7z pre-installed)

Why this fixes the crash: rustler_precompiled (used by mdex_native/Marcli) calls :erlang.system_info(:system_architecture) at compile time and downloads a precompiled NIF for that platform. Building on ubuntu-latest caused the Linux .so to be bundled into macOS and Windows releases, so MDExNative.Native was never available at runtime — crashing lcls --full with an UndefinedFunctionError. Each target now builds on its matching OS, so the correct NIF is fetched and bundled automatically.

Verification: Trigger a release workflow (or workflow_dispatch) and confirm all three legs succeed — especially that lcls --full on macos_aarch64 no longer crashes.

[bougyman]

Rework — address code review

Review issue addressed: Missing `shell: bash` in `burrito-build` `defaults.run`.

Change: Added `shell: bash` to the `defaults.run` block of the `burrito-build` job (.github/workflows/main.yaml line 81). This ensures all run steps in that job use Git Bash on Windows runners rather than the default `pwsh`, making the inline env-var assignment (MIX_ENV=prod BURRITO_TARGET=... mix release lc) and POSIX test syntax in the verify step valid on all three runner OSes.

Commit: `722a65d fix(ci): set shell: bash in burrito-build defaults`

Quality checks: 227/227 tests pass, compilation clean.

No other changes. The `burrito-package` job already runs on `ubuntu-latest` and is unaffected.