Use xcode-select instead

Embark seems to use xcode-select on GitHub Actions to switch to a newer
version of XCode instead of switching the macOS version itself:

https://twitter.com/Ca1ne/status/1372120481147535363

.github/workflows/rust.yml

@@ -469,12 +469,11 @@ jobs:
           # macOS
           - label: macOS aarch64
             target: aarch64-apple-darwin
-            os: macOS-11.0
+            os: macOS-latest
             cross: skip
             tests: skip
             install_target: true
-          # FIXME: Switch OS back to latest once that means 11.
-          # FIXME: Switch OS to ARM macOS once GitHub Actions provides that, 
+          # FIXME: Switch OS to ARM macOS once GitHub Actions provides that,
           # use the toolchain instead of the target and run the tests.
 
           - label: macOS x86_64
@@ -615,6 +614,13 @@ jobs:
         run: sh .github/workflows/install.sh
         env:
           OS_NAME: ${{ matrix.os }}
+          TARGET: ${{ matrix.target }}
+
+      - name: Select XCode to use
+        if: matrix.target == 'aarch64-apple-darwin'
+        run: |
+          sudo xcode-select -s "/Applications/Xcode_12.3.app"
+          sudo rm -Rf /Library/Developer/CommandLineTools/SDKs/*
 
       - name: Build Static Library
         run: sh .github/workflows/build_static.sh