Set _PANTS_OVERRIDE_VERSION when delegate_bootstrap has been overridden.

[kaos]

Special case when running in the Pants source repo and delegate_bootstrap is enabled--when a specific version of Pants is requested (either via pants_version in the pants.toml file or using env vars PANTS_VERSION or PANTS_SHA) bootstrap using that version of Pants rather than delegating to ./pants.

Closes #98

[jsirois]

Excellent, thank you.

[jsirois]

@kaos if you want to try your hand at a release prep / release I'll let you do so; otherwise I'm happy to pick that up presently.

[kaos]

@kaos if you want to try your hand at a release prep / release I'll let you do so; otherwise I'm happy to pick that up presently.

Would be happy to; guessing it is rather straight forward. Today however I'll let you run with it, as it's our anniversary and planning on going out for dinner (16 years married to my wife today 🎉 time flies)

[kaos]

hmm... is it a flaky test to look for the scheduler init string? 😬

[jsirois]

Aha, congratulations! Yeah, I'll hold off on release since there are issues anyhow.

[kaos]

I've put the test lines in a block for readability on a few of the tests to try it out. If you agree with it, I can go over and apply that to all the integration tests. For me, it made it more readable where the different tests started/ended. (also hoping to reduce local variable pollution between tests, although some of the tests rely on it, so those would likely be left outside such a block in that case)