ci(macos): run on pull requests only when macOS setup changes; add weekly run

macOS runners cost roughly 10x Linux per minute, and this workflow was
about $1.8k/month gross running on every pull request. Pull requests now
run it only when they touch the macOS setup scripts, Homebrew pins, pixi
environment, Python requirements or the workflow itself. Pushes to main
still run it on every non-docs change, and a weekly scheduled run catches
upstream Homebrew/conda-forge drift. Non-PR runs save the ccache so pull
requests keep restoring from a fresh entry.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Ramon Roche <mrpollo@gmail.com>

.github/workflows/compile_macos.yml

@@ -6,11 +6,21 @@ on:
     - 'main'
     paths-ignore:
       - 'docs/**'
+  # macOS runners are the most expensive in CI. Pull requests only run this
+  # when they touch the macOS setup; main and the weekly run catch the rest.
   pull_request:
     branches:
       - '**'
-    paths-ignore:
-      - 'docs/**'
+    paths:
+      - 'Tools/setup/macos.sh'
+      - 'Tools/setup/macos_ci_pins.sh'
+      - 'Tools/setup/macos/**'
+      - 'Tools/setup/homebrew-pins.txt'
+      - 'Tools/setup/requirements.txt'
+      - '.github/workflows/compile_macos.yml'
+  schedule:
+    - cron: '0 6 * * 1' # Every Monday at 06:00 UTC
+  workflow_dispatch:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
@@ -63,13 +73,13 @@ jobs:
     - name: Build px4_fmu-v5_default
       run: make px4_fmu-v5_default
 
-    # Only main saves: the 10 GB Actions cache cannot hold an entry for every
-    # PR run, and PRs restore from main's newest entry anyway.
+    # Only non-PR runs save: the 10 GB Actions cache cannot hold an entry for
+    # every PR run, and PRs restore from main's newest entry anyway.
     - name: Cache - Stats
-      if: always() && github.event_name != 'push'
+      if: always() && github.event_name == 'pull_request'
       run: ccache -s
 
     - uses: ./.github/actions/save-ccache
-      if: always() && github.event_name == 'push'
+      if: always() && github.event_name != 'pull_request'
       with:
         cache-primary-key: ${{ steps.ccache.outputs.cache-primary-key }}