ci: Fetch tag before checkout in backcompat test (#61184)

* ci: Fetch tag before checkout in backcompat test

* Specify remote

* Specify --depth=1 to avoid over-fetching

* Test without --depth=1

* fetch tag specifically

---------

Co-authored-by: William Bezuidenhout <william.bezuidenhout@sourcegraph.com>

Author: varungandhi-src

dev/backcompat/bazel-backcompat.sh

@@ -31,6 +31,11 @@ else
   fi
 fi
 
+echo "--- :git: fetch origin v${tag}"
+# For back-compat tests, the shared fetching code doesn't guarantee that
+# the tag has been fetched (because not all tests require tags), so fetch it here.
+git fetch origin tag "v${tag}"
+
 echo "--- :git::rewind: checkout v${tag}"
 # --no-overlay makes so that git ensures the files match what is in the tree exactly, removing files that do not match
 git checkout --force "v${tag}"