CI: Use different git-directory for each runner

It seems that the new approach doesn't go far enough, since when there
are multiple independent runners on a machine, they end up sharing the
same git directory.

Add $CI_RUNNER_ID as well, so they are kept separate.

Series-to: u-boot
Series-cc: trini

Signed-off-by: Simon Glass <sjg@chromium.org>

.gitlab-ci.yml

@@ -5,7 +5,7 @@ variables:
   MIRROR_DOCKER: docker.io
   SJG_LAB: ""
   PLATFORM: linux/amd64,linux/arm64
-  GIT_CLONE_PATH: $CI_BUILDS_DIR/$CI_CONCURRENT_ID/u-boot
+  GIT_CLONE_PATH: $CI_BUILDS_DIR/$CI_RUNNER_ID/$CI_CONCURRENT_ID/u-boot
 
 default:
   tags: