🤖 perf: skip code signing in merge queue to enable parallel builds (#876)

The macOS build workflow was taking 24+ minutes because code signing was
enabled for `merge_group` events. This forced sequential x64+arm64
builds to avoid keychain race conditions.

Since merge queue builds don't need signed artifacts (only releases do),
we can safely skip signing and let both architectures build in parallel.
This should cut the macOS build time roughly in half (~12 min).

**Before:** [24m40s for macOS
build](https://github.com/coder/mux/actions/runs/19892016638/job/57012954221)

**After:** Expected ~12 min (parallel x64+arm64 builds)

---
_Generated with `mux`_

Author: ibetitsmike

.github/workflows/build.yml

@@ -18,11 +18,12 @@ 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).
+      # Skip signing setup for PRs and merge queue - only release builds need signing.
+      # Setting CSC_LINK forces our Makefile to build sequentially (to avoid keychain
+      # race conditions), which doubles build time. Without it, x64 and arm64 build
+      # in parallel (~12 min savings).
       - name: Setup code signing
-        if: github.event_name != 'pull_request'
+        if: github.event_name == 'workflow_dispatch'
         run: ./scripts/setup-macos-signing.sh
         env:
           MACOS_CERTIFICATE: ${{ secrets.MACOS_CERTIFICATE }}
@@ -32,7 +33,7 @@ jobs:
           AC_APIKEY_ISSUER_ID: ${{ secrets.AC_APIKEY_ISSUER_ID }}
 
       - name: Verify signing setup
-        if: github.event_name != 'pull_request'
+        if: github.event_name == 'workflow_dispatch'
         run: |
           if [ -n "${CSC_LINK:-}" ]; then
             echo "✅ Code signing enabled"