manifest: Use chromiumos-overlay from third_party.

Currently, R77 and R81 uses chromiumos-overlay from
https://cos.googlesource.com/mirrors/cros/chromiumos/overlays/chromiumos-overlay
where R85 and above uses from
https://cos.googlesource.com/third_party/overlays/chromiumos-overlay/

This creates if-else cases in the release notes tool while generating
changelog for chromiumos-overlay as the repo names are different.
As the commits currently used for R77 and R81 are present in
https://cos.googlesource.com/third_party/overlays/chromiumos-overlay/,
change the repo to
https://cos.googlesource.com/third_party/overlays/chromiumos-overlay/
so that all the branches are using the same repo and it avoids the
if-else condition for generating changelog.

BUG=b/172264418
TEST=repo init and sync

Change-Id: Iabc75a86133c0c8b274223e4bc9bf8f114b36c2f
Reviewed-on: https://cos-review.googlesource.com/c/cos/manifest/+/8282
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Tested-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
diff --git a/external_full.xml b/external_full.xml
index 73082f6..9ba341d 100644
--- a/external_full.xml
+++ b/external_full.xml
@@ -44,7 +44,7 @@
            revision="4b5b686c5d1892910d6826bd11338837fd9c39cf" />
   <project path="src/third_party/chromiumos-overlay"
            sync-c="true"
-           name="mirrors/cros/chromiumos/overlays/chromiumos-overlay"
+           name="third_party/overlays/chromiumos-overlay"
            revision="bc0b61b2dcced141dfcd6d750bfcce2866618003" />
   <project path="src/third_party/eclass-overlay"
            name="mirrors/cros/chromiumos/overlays/eclass-overlay"