Use DEVELOPER_DIR instead of a custom xcode-select script

`DEVELOPER_DIR` is the standard environment variable for overriding the
Xcode version, there is no need to invoke `xcode-select --switch`
manually to do this.

The variable is documented in both `man xcode-select` and `man xcrun`.

.github/workflows/ci.yml

@@ -165,9 +165,6 @@ jobs:
       - name: install sccache
         run: src/ci/scripts/install-sccache.sh
 
-      - name: select Xcode
-        run: src/ci/scripts/select-xcode.sh
-
       - name: install clang
         run: src/ci/scripts/install-clang.sh