GHA: Sync main.yml with DMD repo's

.github/workflows/main.yml

@@ -40,7 +40,7 @@ jobs:
       OS_NAME: ${{ startsWith(matrix.os, 'ubuntu') && 'linux' || (startsWith(matrix.os, 'macos') && 'osx' || (startsWith(matrix.os, 'windows') && 'windows' || '')) }}
       MODEL: ${{ matrix.model || '64' }}
       HOST_DMD: dmd
-      N: ${{ startsWith(matrix.os, 'macos') && '3' || '4' }}
+      # N is set dynamically below
       FULL_BUILD: false
       # work around https://issues.dlang.org/show_bug.cgi?id=23517
       MACOSX_DEPLOYMENT_TARGET: '11'
@@ -71,6 +71,9 @@ jobs:
 
           git clone --branch "$REPO_BRANCH" --depth 1 https://github.com/dlang/dmd.git ../dmd
 
+      - name: Set environment variable N (parallelism)
+        run: echo "N=$(${{ runner.os == 'macOS' && 'sysctl -n hw.logicalcpu' || 'nproc' }})" >> $GITHUB_ENV
+
       - name: 'Posix: Install prerequisites'
         if: runner.os != 'Windows'
         run: cd ../dmd && ${{ runner.os == 'macOS' && 'ci/cirrusci.sh' || 'sudo -E ci/cirrusci.sh' }}
@@ -80,6 +83,13 @@ jobs:
         with:
           arch: ${{ env.MODEL == '64' && 'x64' || 'x86' }}
 
+      # NOTE: Linker ICEs with Xcode 15.0.1 (default version on macos-13)
+      # * https://issues.dlang.org/show_bug.cgi?id=24407
+      # Remove this step if the default gets changed to 15.1 in actions/runner-images.
+      - name: 'macOS 13: Switch to Xcode v15.1'
+        if: matrix.os == 'macos-13'
+        run: sudo xcode-select -switch /Applications/Xcode_15.1.app
+
       - name: 'Posix: Install host compiler'
         if: runner.os != 'Windows'
         run: cd ../dmd && ci/run.sh install_host_compiler
@@ -127,6 +137,7 @@ jobs:
           operating_system: freebsd
           hypervisor: qemu
           memory: 12G
+          cpu_count: 4
           sync_files: runner-to-vm
           version: ${{ matrix.freebsd_version }}
           shell: bash
@@ -139,7 +150,7 @@ jobs:
             export OS_NAME=freebsd
             export MODEL=64
             export HOST_DMD=dmd
-            export N=3
+            export N="$(nproc)"
             export FULL_BUILD=false
             export CI_DFLAGS="-version=TARGET_FREEBSD${freebsd_major}"