rustbuild: save-analysis inadvertently nuked

[xen0n]

So #38631 invalidated the assumption that the final stage is stage2, but this piece of code still thought so, so the save-analysis data is never generated. This enraged the bots.

Let's polish rustbuild with some more <3 so that we could have a fresh nightly for a nice New Year gift to everyone...

[xen0n]

BTW the failure on the win32 beta bot (ouch!) is not related to this. It's most likely a Windows hiccup.

[xen0n]

Ahhhh big failure of mine. I'm focusing on the generation part (the envvar) and forgot to touch the dist.rs logic too, and the bots are as red as yesterday (facepalm). Trying to hit the 2017-01-02 nightly instead...