🤖 ci: parallelize macOS PR builds by skipping signing setup

PR builds were running x64 and arm64 sequentially (~180s) because:
1. CSC_LINK was set by the signing setup script
2. Makefile sees CSC_LINK and forces sequential builds to avoid keychain conflicts
3. But electron-builder skips signing for PRs anyway!

Fix: Skip signing setup for PRs entirely. This allows the Makefile to
run both architectures in parallel, saving ~90 seconds per PR build.

Evidence from run #19845830777:
- 03:24:40 "🔐 Code signing enabled - building sequentially..."
- 03:26:12 "Current build is a part of pull request, code signing will be skipped"
- x64: 03:26:12 → 03:27:43 (~91s)
- arm64: 03:27:59 → 03:29:29 (~90s) - started AFTER x64 finished

With this change, both will build simultaneously.

_Generated with mux_

Author: ammar-agent

.github/workflows/build.yml

@@ -18,7 +18,11 @@ jobs:
 
       - uses: ./.github/actions/setup-mux
 
+      # Skip signing setup for PRs - electron-builder skips signing anyway,
+      # but setting CSC_LINK forces our Makefile to build sequentially.
+      # Without CSC_LINK, x64 and arm64 build in parallel (~90s savings).
       - name: Setup code signing
+        if: github.event_name != 'pull_request'
         run: ./scripts/setup-macos-signing.sh
         env:
           MACOS_CERTIFICATE: ${{ secrets.MACOS_CERTIFICATE }}
@@ -28,6 +32,7 @@ jobs:
           AC_APIKEY_ISSUER_ID: ${{ secrets.AC_APIKEY_ISSUER_ID }}
 
       - name: Verify signing setup
+        if: github.event_name != 'pull_request'
         run: |
           if [ -n "${CSC_LINK:-}" ]; then
             echo "✅ Code signing enabled"
@@ -39,8 +44,6 @@ jobs:
 
       - name: Package for macOS
         run: make dist-mac
-        env:
-          CSC_FOR_PULL_REQUEST: ${{ github.event.pull_request.number == 234 }}
 
       - name: Upload macOS DMG (x64)
         uses: actions/upload-artifact@v4