Skip platform-specific builds based on formula dependencies

[Copilot]

Problem

When a PR contains formulas with platform-specific dependencies (e.g., depends_on :linux), the CI workflow was unnecessarily running builds on all platforms. As shown in the issue screenshot, a PR with the "linux-only" label still triggered all 4 macOS runners (macos-14, macos-15, macos-15-intel, macos-26) even though they would never succeed for a Linux-only formula.

This wastes CI resources, increases queue times, and delays feedback for contributors.

Solution

This PR implements intelligent platform filtering in the CI workflow, matching the behavior of homebrew/homebrew-core:

• Linux-only PRs (formulas with depends_on :linux): Skip all macOS runners, only run Linux builds
• macOS-only PRs (formulas with depends_on :macos): Skip all Linux runners, only run macOS builds
• Multi-platform PRs: Run all platforms as before

Implementation

1. Label Detection (environment.js)

Extended the environment configuration script to detect linux-only and macos-only labels (which are automatically applied by the triage.yml workflow) and set corresponding output flags.

2. Conditional Runner Execution (tests.yml)

• Added skip_on_linux_only and skip_on_macos_only flags to each matrix runner
• Introduced a skip-check step that evaluates whether a runner should execute based on:
  • Event type (only applies to pull requests)
  • Label presence (linux-only or macos-only)
  • Runner platform (macOS vs Linux)
• Made all build steps conditional on the skip flag

Benefits

• Resource Optimization: Up to 66% reduction in CI runs for linux-only PRs
• Cost Savings: Significantly fewer GitHub Actions minutes, especially for expensive macOS runners (10x cost of Linux)
• Faster Feedback: Quicker turnaround for contributors on platform-specific PRs
• Consistency: Aligns with Homebrew core tap practices

Example

For a PR adding a Linux-only formula:

Before:

• 4 macOS runners queued → will fail ❌
• 2 Linux runners queued → will succeed ✅
• Total: 6 runners, 4 unnecessary

After:

• 4 macOS runners skipped ⏭️
• 2 Linux runners queued → will succeed ✅
• Total: 2 runners, 0 unnecessary

Safety

• Non-PR events (push to main) always run all platforms
• Missing labels default to running all platforms
• No impact on existing multi-platform formulas
• All changes validated with YAML and JavaScript syntax checks

Fixes #[issue_number]

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[bug] like homebrew core tap, the linux-only prs should not trigger macos builds</issue_title>
<issue_description>

• httptap: rebuild #2005</issue_description>

<agent_instructions>need to update the workflow to not create macos builds run for linux-only builds</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #2007

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