tests: run osbuild as a python module in gen-stage-test-diff

Instead of running osbuild as a binary use `python3 -m osbuild`
(just like in `test/test.py:compile()`) so that it will use
osbuild fromgit and can be run from a checkout without the need
for an installed osbuild.

tools/gen-stage-test-diff

@@ -8,7 +8,7 @@ import tempfile
 
 def run_osbuild(output_directory, store, cache_max_size, libdir, manifest):
     args = [
-        "osbuild",
+        "python3", "-m", "osbuild",
         "--export",
         "tree",
         "--output-directory",