actions: update homebrew qt package name

qt5 seems to have been renamed to qt@5 in homebrew, so builds were
failing at the CMAKE_PREFIX_PATH was no longer correct.

.github/workflows/build.yml

@@ -5,7 +5,7 @@ on: [push]
 env:
   BUILD_TYPE: Release
   # For macOS qt keg-only package
-  CMAKE_PREFIX_PATH: '/usr/local/opt/qt'
+  CMAKE_PREFIX_PATH: '/usr/local/opt/qt@5'
 
 jobs:
   build:
@@ -18,7 +18,7 @@ jobs:
     - uses: actions/checkout@v2
 
     - name: Install dependencies (macOS)
-      run: brew install fftw liquid-dsp qt5
+      run: brew install fftw liquid-dsp qt@5
       if: matrix.os == 'macos-latest'
 
     - name: Install dependencies (Ubuntu)