ci: fix macOS runner

It seems the GitHub Actions macOS image has a conflicting python in
/usr/local, invoke:

brew -v install python
brew link --overwrite python

, before installing dependencies.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

installdeps

@@ -1199,6 +1199,10 @@ brew_installdeps() {
         brews="$brews sfml"
 #    fi
 
+    # This is necessary for the GitHub Actions CI:
+    brew -v install python
+    brew link --overwrite python
+
     check brew -v install -f $brews
 
     for brew in gettext wxwidgets; do