[v12] build: Copy release artifacts to release directory

[camscale]

Copy the tarball and mac packages/dmgs release artifacts to a release
artifact directory so that the CI scripts do not have to. This makes it
clearer what is and is not a release artifact and puts the logic in the
Makefile instead of the CI yaml, so it can more easily be tested locally
and to make it easier to migrate to the next CI system.

Backport: #25460 (partial)
Companion: https://github.com/gravitational/teleport.e/pull/2804

---

This is a partial backport - just the first commit - of #25460.This PR
added universal mac builds which was not backported to v12. However it
also added the RELEASE_DIR to the Makefile which is needed for the
GitHub Actions workflow, so I'm backporting just that commit.

I will likely also update the e ref in the PR once the companion PR is merged.

[ravicious]

Do we need to check CONNECT_TSH_APP_PATH before copying the dmg? The comment above release-connect says that this target is used only in macOS, in which case the dmg should always be present after a successful yarn package-term.

[camscale]

This is discriminating between CONNECT_TSH_APP_PATH and CONNECT_TSH_BIN_PATH. The former is set when we are building release packages. On a push build we don't care about saving the package, and that uses CONNECT_TSH_BIN_PATH.

https://github.com/gravitational/teleport.e/blob/branch/v12/.github/workflows/build-mac-amd64.yaml#L193 and the step after the highlighted line.

I will shortly be revisiting the whole copying artifact thing when we stop using Drone to do any of this. I may revise this logic then to be more straightforward. But for now I'll keep the branches in line with each other as much as I can.

[ravicious]

Oops, I forgot that it does push builds too, I thought it's just for tag builds.