Make it clear we're running clang-12 for the GH actions

This chases a homebrew change in the default clang version.

Reviewed by:	arichardson
Differential Revision: https://reviews.freebsd.org/D31231

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

@@ -23,10 +23,10 @@ jobs:
             compiler: clang-10
             cross-bindir: /usr/lib/llvm-10/bin
             pkgs: bmake libarchive-dev clang-10 lld-10
-          - os: macOS-latest
-            compiler: clang-11
+          - os: macos-latest
+            compiler: clang-12
+            cross-bindir: /usr/local/opt/llvm/bin
             pkgs: bmake libarchive
-
     steps:
       - uses: actions/checkout@v2
       - name: install packages (Ubuntu)