CI: Disable introspection when building with clang

f28 default cflags are picked up by g-ir-scanner and they don't work with clang as per:
 https://bugzilla.redhat.com/show_bug.cgi?id=1543295

Closes: #1741
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f