minstall: do not drop privileges if msetup also ran under sudo

A user might run `sudo somewrapper` to build and install something with
meson, and it is not actually possible to drop privileges and build,
since the build directory is also owned by root.

A common case of this is `sudo pip install` for projects using
meson-python or other python build-backends that wrap around meson.

Fixes #11665

mesonbuild/minstall.py

@@ -788,6 +788,10 @@ def rebuild_all(wd: str, backend: str) -> bool:
             else:
                 return None, None
 
+            if os.stat(os.path.join(wd, 'build.ninja')).st_uid != int(orig_uid):
+                # the entire build process is running with sudo, we can't drop privileges
+                return None, None
+
             env['USER'] = orig_user
             env['HOME'] = homedir