Uppercase MATRIX_NAME env var in skaffold repo step

[shikanime]

What

The repo step in modules/devenv/integrations/github/skaffold.nix sets a step
env var consumed inside the bash body. GitHub Actions step env keys are
conventionally uppercase, so rename it to MATRIX_NAME for consistency with
the rest of the workflow's output names.

• env.matrix_name → env.MATRIX_NAME
• read site updated: ''${MATRIX_NAME:-$name}

Why

Aligns the inlined repo step with GitHub Actions naming conventions; no
behavioral change beyond the env key casing.

Related: #402

Summary by CodeRabbit

• Bug Fixes
  • Corrected profile-specific manifest repository selection by ensuring the matrix profile is applied consistently.
  • Preserved the existing generated repository path format.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 72b0b01c-0d44-447e-9ab2-982de234cfa7

📥 Commits

Reviewing files that changed from the base of the PR and between 4f7c0c9 and 264a4b3.

📒 Files selected for processing (1)

• modules/devenv/integrations/github/skaffold.nix

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📝 Walkthrough

Walkthrough

The profile-specific repository step now exports the matrix profile as MATRIX_NAME and uses it for manifest repository fallback selection.

Changes

Profile repository selection

Layer / File(s)	Summary
Update profile repository selection modules/devenv/integrations/github/skaffold.nix	The repository step renames matrix_name to MATRIX_NAME and uses the uppercase variable when selecting the manifest repository name.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to 264a4

This change only standardizes the step environment variable name and its read site, with no actionable merge-blocking risk remaining beyond normal checks and review.

Suggested reviewers: yorha-operator

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: renaming the Skaffold repository step environment variable to uppercase MATRIX_NAME.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/skaffold-repo-env-upper

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