GitHub Actions: Build macOS dmg file only on release

The regular builds should be as fast as possible.

Change-Id: I76b9ac009831192053f2163b1f73fb2183f3e791
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

.github/workflows/build_cmake.yml

@@ -483,8 +483,10 @@ jobs:
         string(REPLACE "x86" "x64" Python3_EXECUTABLE "${Python3_EXECUTABLE}")
 
         set(WITH_TESTS "--with-tests")
+        set(NO_DMG "--no-dmg")
         if (${{github.ref}} MATCHES "tags/v")
           unset(WITH_TESTS)
+          unset(NO_DMG)
         endif()
 
         execute_process(
@@ -500,6 +502,7 @@ jobs:
             ${WITH_TESTS}
             ${CDB_OPTION}
             ${ELFUTILS_OPTION}
+            ${NO_DMG}
             --add-config=-DCMAKE_C_COMPILER_LAUNCHER=ccache
             --add-config=-DCMAKE_CXX_COMPILER_LAUNCHER=ccache
             --add-config=-DIDE_REVISION_URL=https://github.com/$ENV{GITHUB_REPOSITORY}/commits/$ENV{GITHUB_SHA}
@@ -588,7 +591,7 @@ jobs:
         name: qtcreatorcdbext-${{ matrix.config.artifact }}-${{ github.run_id }}.7z
 
     - name: Upload disk image
-      if: runner.os == 'macOS'
+      if: runner.os == 'macOS' && contains(github.ref, 'tags/v')
       uses: actions/upload-artifact@v2
       with:
         path: build/qt-creator-${{ matrix.config.artifact }}-${{ github.run_id }}.dmg