ci: Change owner of files in source-tarball job

This fixes errors of the form:

```
fatal: detected dubious ownership in repository at '/builds/ghc/ghc'
To add an exception for this directory, call:
	git config --global --add safe.directory /builds/ghc/ghc
inferred 9.7.20230113
checking for GHC Git commit id... fatal: detected dubious ownership in repository at '/builds/ghc/ghc'
To add an exception for this directory, call:
	git config --global --add safe.directory /builds/ghc/ghc
```

.gitlab-ci.yml

@@ -581,6 +581,7 @@ source-tarball:
     paths:
       - ghc-*.tar.xz
   script:
+    - sudo chown ghc:ghc -R .
     - ./boot
     - ./configure
     - ./hadrian/build source-dist