GitHub Actions: Specify Big Sur for macOS build

The Catalina hosted runner is now fully deprecated.

.github/workflows/build.yml

@@ -51,7 +51,7 @@ jobs:
         run: |
           aws s3 sync --acl public-read --delete $HOME/src/ljt.nightly/latest/files/ s3://libjpeg-turbo-pr/${{env.BRANCH}}/${{github.job}}/
   macos:
-    runs-on: macos-10.15
+    runs-on: macos-11
     steps:
       - name: Set global environment variables
         run: |
@@ -61,6 +61,7 @@ jobs:
       - name: Set up build
         run: |
           brew install yasm md5sha1sum
+          sudo xcode-select -s /Applications/Xcode_12.4.app
           mkdir -p $HOME/src/ljt.nightly
           git clone --depth=1 https://github.com/libjpeg-turbo/buildscripts.git -b $BRANCH $HOME/src/buildscripts
       - name: Configure GPG signing