Rollup merge of #41047 - cuviper:src_is_git, r=alexcrichton

Only use cargo-vendor if building from git sources

The only time we need to vendor sources is when building from git.  If one is
building from a rustc source tarball, everything should already be in place.
This also matters for distros which do offline builds, as they can't install
cargo-vendor this way.

This adds a common `Build::src_is_git` flag, and then uses it in the dist-src
target to decide whether to install or use `cargo-vendor` at all.

Fixes #41042.