rustbuild shouldn't require cargo-vendor when building from tarballs

[cuviper]

Rustbuild uses cargo-vendor to get complete sources for the dist-src target, but this isn't needed when building from tarballs rather than git, as everything is already vendored. It breaks distro builds when it tries to install cargo-vendor, as we have no network access allowed. I see this on 1.17-beta, but 1.16 didn't deal with cargo-vendor.

I don't really even need the dist-src target anyway, as I just want to install binaries and docs, but ./x.py dist --install always requires it AFAICT, even when explicit paths are specified.
(e.g. ./x.py dist --install src/librustc src/libstd src/doc)

[est31]

cc @alexcrichton

[cuviper]

I was talking to @alexcrichton about this on IRC already. I have a PR for this ready in just a moment.

[arielb1]

This sounds like a regression? (should we beta-nominate the fix)?