ci: Select explicit Xcode version on macOS

Github has bumped the default version for Xcode to one that is not
officially supported by the current version of Qt, and thus causes
warnings (and potentially other issues down the line).

Avoid this by explicitly selecting Xcode 10.3, which is currently the
oldest version provided by Github actions.

diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 125b99b..c0dae8e 100644 (file)
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -93,6 +93,12 @@ jobs:
       CCACHE_MAXSIZE: 100M
 
     steps:
+    - name: Select Xcode version
+      uses: maxim-lobanov/setup-xcode@v1
+      with:
+        # Newer Xcode versions may not officially be supported by Qt
+        xcode-version: '10.3'
+
     - name: Check out source code
       uses: actions/checkout@v2
       with: