Fix building of rubygems

[DRMacIver]

Two Three problems were occurring:

1. Silly error on my part where I wrote os.path.unlink instead of os.link. @alexwlchan based on your suggestion in Build and release a Conjecture crate #1339 I've ended up extracting a function to the junk drawer for safe unlinking as part of this.
2. Due to hilarious comedy, the change I made in Factor out Python specific build components #1332 for more robust checking for an install of the build environment meant that (on Travis only) we reran all failing jobs, because the TASK environment variable was overriding the check-installed command we were explicitly passing.
3. Our build was depending on rust nightly and thus hitting rustup can't update nightly rust-lang/rust#51699. I'm not sure why I went with nightly rather than stable (the library is definitely meant to work on stable!) so I've just switched this over to stable.

[DRMacIver]

@alexwlchan Any objections to my skipping the long build and using admin powers to merge this on grounds that master is already broken so this will either go green on master or do no further harm?

[DRMacIver]

(I decided not to wait for objections, because I really can't think of anything that a green build of the rest of this would tell us)

[DRMacIver]

Correct call. It broke in a way that this build wouldn't have detected! Sigh.

I've pushed a hot fix straight to master. 244e829