Make pr_preview.py pass the token as a password rather than a username (#26894)

It seems like the code was passing the token as a username rather than a password,
which should never work. (and indeed refs weren't being deleted). Try setting a username
and using it as the password to see if that works.
diff --git a/tools/ci/pr_preview.py b/tools/ci/pr_preview.py
index 5bd2365..369abf5 100755
--- a/tools/ci/pr_preview.py
+++ b/tools/ci/pr_preview.py
@@ -217,9 +217,9 @@
         return subprocess.check_output([
             'git',
             '-c',
-            'credential.username={}'.format(self._token),
+            'credential.username=github-actions',
             '-c',
-            'core.askPass=true',
+            'credential.password={}'.format(self._token),
         ] + command)
 
     def get_revision(self, refspec):