.github: Use make.py's --cross-toolchain option on macOS

Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D51749

.github/workflows/cross-bootstrap-tools.yml

@@ -31,7 +31,7 @@ jobs:
             pkgs: bmake libarchive-dev clang-18 lld-18
           - os: macos-latest
             compiler: clang-18
-            cross-bindir: /opt/homebrew/opt/llvm@18/bin
+            cross-toolchain: llvm@18
             pkgs: bmake libarchive llvm@18
           - target_arch: amd64
             target: amd64
@@ -55,6 +55,9 @@ jobs:
           if [ -n "${{ matrix.cross-bindir }}" ]; then
             echo "EXTRA_BUILD_ARGS=--cross-bindir=${{ matrix.cross-bindir }}" >> $GITHUB_ENV
           fi
+          if [ -n "${{ matrix.cross-toolchain }}" ]; then
+            echo "EXTRA_BUILD_ARGS=--cross-toolchain=${{ matrix.cross-toolchain }}" >> $GITHUB_ENV
+          fi
           mkdir -p ../build
           echo "MAKEOBJDIRPREFIX=${PWD%/*}/build" >> $GITHUB_ENV
           # heh, works on Linux/BSD/macOS ...