Drop Qt version slightly

Signed-off-by: Matt Milner <[email protected]>

.github/workflows/build_cmake.yml

@@ -7,7 +7,7 @@ name: CMake Build Matrix
 on: [push, pull_request, workflow_dispatch]
 
 env:
-  QT_VERSION: 5.15.15
+  QT_VERSION: 5.15.8
   # this is different from MACOSX_DEPLOYMENT_TARGET to prevent build problems
   # we set MACOSX_DEPLOYMENT_TARGET later
   MACOS_TARGET: 10.12