Add _lcov_merger attribute to test rules

[keith]

As of bazel 5.1 this can be used as a late bound attribute bazelbuild/bazel@14d8be0

This alleviates the concerns from
#691 and allows users to
use this feature, and avoid having to pass LCOV_MERGER in the
environment manually

[lyft-lint-bot]

Lyft integration job started: https://buildkite.com/lyft/rules-apple/builds/444 (must be Lyft employee to view)

[lyft-lint-bot]

Lyft integration job started: https://buildkite.com/lyft/rules-apple/builds/486 (must be Lyft employee to view)

[lyft-lint-bot]

Lyft integration job started: https://buildkite.com/lyft/rules-apple/builds/633 (must be Lyft employee to view)

[brentleyjones]

I would cut one more release before merging this, but then I think it's 👍

[keith]

Do we need a new release first even? since it's in bazel 5.1+? I think of us supporting LTS meaning the newest in the LTS release version

[brentleyjones]

My thoughts are:

• We have over 80 commits since the last release
• This is a breaking change for some people, who can't go to 5.1 for some reason
• So why not at least give them the latest changes as a release? We can even do two releases back to back if we want (maybe the second one with this has the new versioning we've been thinking about)?

It doesn't directly impact me, since I think anyone on 5.x should be on 5.2 right now, but that was my reasoning at least.

[keith]

ok since we've done a release since then we can probably merge this one now?