swarmbucket: git_cache must be in cwd

on swarmbucket, git_cache must be in cwd

R=vadimsh@chromium.org, martiniss@chromium.org

Review-Url: https://codereview.chromium.org/2473243003

recipe_modules/infra_paths/path_config.py

@@ -55,4 +55,4 @@ def infra_kitchen(c):
 
 @CONFIG_CTX(includes=['infra_buildbot'])
 def infra_swarmbucket(c):
-  c.base_paths['git_cache'] = c.base_paths['root'] + ('git_cache',)
+  c.base_paths['git_cache'] = c.CURRENT_WORKING_DIR + ('git_cache',)