Add retries to GitHub land API call

[jozef-mokry]

In Cord we notice that when working on a stack of diffs, then
sometimes spr land fails with the following error:

  ❌  GitHub Pull Request merge failed
  🛑  GitHub: At least 1 approving review is required by reviewers with write access.
      Documentation URL: https://docs.github.com/articles/about-protected-branches

This problem goes away if we go to the PR and manually change the PR's
base branch to master. Then spr land works fine. This is surprising
because in the PR history I can see that spr changed the base branch to
master too when we tried to spr land the first time.

I initially added a 5s sleep call, and the problem went away. Hence, I
suspect this is an issue on GitHub's end. Hence, I added a retry
mechanism around the API call.

I added a utility function do_with_retry() to do this. Not as much for
code re-use, but because it makes the code easier to read IMO. There is
code above my changes that also uses retries, but I did not refactor it
to use do_with_retry() because that code looked more complex 🙃

Test Plan: I will be using spr with this change for the next few days

[jwatzman]

@jozef-mokry do we still need this? If so, looks reasonable to me. I don't know enough Rust machinery to know what the build failure is about but I assume you do :)

If we don't need this, can just close out?