chore(ci): shard //internal/repos:repos_test (#64412)

See
https://buildkite.com/organizations/sourcegraph/analytics/suites/sourcegraph-bazel/tests/d047e8dd-c849-8a8b-8925-41c2b8e58ec9?branch=main,
this particular test target tends to timeout on occasions, because it
goes over the `short` timeout.

By sharding it, we could hope it would be more stable. So, time will
tell if that was worth it. Right now, analytics shows that it goes down
to 10s with four shards, which is faster that most runs.

## Test plan

CI

Author: jhchabran

internal/repos/BUILD.bazel

@@ -144,6 +144,7 @@ go_test(
     ],
     data = glob(["testdata/**"]),
     embed = [":repos"],
+    shard_count = 4,
     tags = [
         TAG_PLATFORM_SOURCE,
         # Test requires localhost database