Pants-from-source-from-git-URI

[tgolsson]

I've had a use-case where I want to test a fork of Pants in CI. This isn't too hard to do by adding a second checkout and setting workdirs. However; it'd be neater if I could f.ex. say PANTS_SOURCE=git+https://github.com/tgolsson/pants@my-branch and have it download, build, and run the Pants given by that commit.