[DNM] Update swift_package_test.yml

Trying to see what Xcodes are present on the builders and why we're seeing failures to find Xcode downstream.

Author: grynspan

.github/workflows/swift_package_test.yml

@@ -180,6 +180,8 @@ jobs:
             do
               printf "%s\n" $i >> $GITHUB_ENV
             done
+      - name: List installed applications.
+        run: ls /Applications
       - name: Select Xcode
         run: echo "DEVELOPER_DIR=/Applications/Xcode_${{ matrix.xcode_version }}.app" >> $GITHUB_ENV
       - name: Swift version