rustbuild: Fix source tarballs and the vendor dir
rustbuild: Fix source tarballs and the vendor dir

The source tarball creation step would attempt to skip a number of files that we
want to ignore ourselves, but once we've hit the vendor directory we don't want
to skip anything so be sure to vendor everything inside that directory.

Closes #38690

aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust