ci: fix test-file-plug-ins job

This job fails because our test plug-ins isn't installed in stable or
release versions. However we are in a RC1 status which leaves stable
and release true.

Let's fix this by checking for "+git" at the end of the version string
and also installing our plug-in if that is found.

plug-ins/python/tests/test-file-plug-ins/meson.build

@@ -1,4 +1,4 @@
-if not stable or not release
+if not stable or not release or gimp_version.endswith('+git')
 
 # This does not get installed for releases
 plugins += {