test: travis: cache stage0

[matthiaskrgr]

This is a test to see if we can cache stage0.
cc #48412

edit:
log 1: https://travis-ci.org/rust-lang/rust/builds/349703962
I force pushed to see if I could see any effect of this in the second log, but I am not sure if this will work / if the cache is retained per PR or if other PR builds that happened in between overwrote the cache.
log 2: https://travis-ci.org/rust-lang/rust/builds/349737662

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[alexcrichton]

Thanks for the PR! Unfortunately though we've had pretty bad luck with caches and Travis historically. They tend to not work well across OSX/Linux/AppVeyor as they increase in size and only very irregularly get cleaned.

That being said the next-best solution we have is sccache which has historically been much more reliable and should perform similarly!

[matthiaskrgr]

Ok, closing.