Xcodebuild Issues using parallel builds + Shell executor

After upgrading to Xcode 12.4, the build jobs have stopped working for me. My only clue is that the new version uses parallel builds and the main process is probably replaced by other child processes and therefore the job always fails unexpectedly shortly after it’s started without any error messages whatsoever! Here’s as far as it gets:

$ xcodebuild -scheme MyLibrary -destination generic/platform=tvOS -allowProvisioningUpdates clean analyze
Command line invocation:
    /Applications/Xcode.app/Contents/Developer/usr/bin/xcodebuild -scheme MyLibrary -destination generic/platform=tvOS -allowProvisioningUpdates clean analyze
2021-03-12 01:18:29.252 xcodebuild[5978:97561] CFURLRequestSetHTTPCookieStorageAcceptPolicy_block_invoke: no longer implemented and should not be called
Resolve Package Graph
Resolved source packages:
  MyLibrary: /usr/local/bin/builds/17a6bd1d/0/my-library
  Alamofire: https://github.com/Alamofire/Alamofire @ 5.4.1
  DataCompression: https://github.com/mw99/DataCompression @ 3.6.0
  SwiftyJSON: https://github.com/SwiftyJSON/SwiftyJSON @ 5.0.0
  Siesta: https://github.com/bustoutsolutions/siesta @ 1.5.2
note: Using new build system
note: Building targets in parallel

This works perfectly on my local machine as well as on the same machine GitLab Runner executes it.

Hah, upgrading the gitlab-runner from 10.8.0 to 13.9.0 solved the issues!