Enforce Homebrew Python on Intel macOS runners

It appears that GitHub runners come with an extra "framework" Python
symlinked into /usr/local. On arm64 runners this has no effect, since
the Homebrew prefix has changed to /opt/homebrew. On x86_64 runners,
however, Homebrew still lives in /usr/local and this is probably why
CMake can't find the right Python.

Try the minimum workaround first.

.github/workflows/osx.yml

@@ -18,18 +18,22 @@ jobs:
     continue-on-error: true
 
     steps:
-      - name: checkout
+      - name: Checkout
         uses: actions/checkout@v4
         with:
           # Needed for `setuptools-scm`
           fetch-depth: 0
 
-      - name: install homebrew packages
+      # See https://github.com/orgs/Homebrew/discussions/3895#discussioncomment-4130560
+      - name: Ensure we use Homebrew Python
+        run: find /usr/local/bin -lname '*/Library/Frameworks/Python.framework/*' -delete
+
+      - name: Install Homebrew packages
         run: |
           brew tap casacore/tap 
           brew install casacore
 
-      - name: make virtualenv
+      - name: Make virtualenv
         # XXX Use Homebrew's numpy to be safe for now, especially since
         # PyPI is now on numpy 2.0, which complicates matters
         run: python3 -m venv venv --system-site-packages
@@ -43,7 +47,7 @@ jobs:
       - name: Run pytest
         run: cd tests && ../venv/bin/pytest
 
-      - name: make binary wheel
+      - name: Make binary wheel
         run: venv/bin/pip wheel -w dist .
 
       - name: Delocate binary wheel