iotests: get rid of '..' in path environment output

Resolve the build_root before we append more items onto it so that the
environment output is more concise with less parent directory confetti
in it.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20230621002121.1609612-4-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/qemu-iotests/testenv.py

@@ -216,7 +216,7 @@ class TestEnv(ContextManager['TestEnv']):
         self.source_iotests = source_dir
         self.build_iotests = build_dir
 
-        self.build_root = os.path.join(self.build_iotests, '..', '..')
+        self.build_root = Path(self.build_iotests).parent.parent
 
         self.init_directories()