Fix test: use formula-specific prefix for libexec check

The test was failing because $(brew --prefix) returns the global prefix
(/opt/homebrew), but libexec is formula-specific and only accessible via
$(brew --prefix mfc) which resolves to /opt/homebrew/opt/mfc.

Binaries in bin/ and files in share/mfc/ are correctly accessible from
the global prefix since Homebrew links them there.

Author: sbryngelson

.github/workflows/homebrew.yml

@@ -93,7 +93,7 @@ jobs:
           echo "  ✓ All binaries exist and are executable"
           
           echo "2. Verifying installation structure..."
-          test -f $(brew --prefix)/libexec/mfc.sh
+          test -f $(brew --prefix mfc)/libexec/mfc.sh
           test -d $(brew --prefix)/share/mfc/toolchain
           test -d $(brew --prefix)/share/mfc/toolchain/mfc
           echo "  ✓ Installation structure verified"