Switch to absolute refs in recipes.cfg.

Using a non-absolute ref results in deprecation warnings in the logs
when downstream repos run.

Change-Id: Ib72bda258b7155cd2081d7ddf7112d6b94f4f015
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3024691
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>

infra/config/recipes.cfg

@@ -15,7 +15,7 @@
   "canonical_repo_url": "https://chromium.googlesource.com/chromium/tools/depot_tools.git",
   "deps": {
     "recipe_engine": {
-      "branch": "main",
+      "branch": "refs/heads/main",
       "revision": "c442abe17a96f858f281b0b5a597cd5d577b4526",
       "url": "https://chromium.googlesource.com/infra/luci/recipes-py.git"
     }