Rollup merge of #125648 - lqd:rustsrc, r=pietroalbini

Remove unused(?) `~/rustsrc` folder from docker script

Every time I run a docker image, the script creates an empty `~/rustsrc` folder. It doesn't seem to be referenced anywhere else, so I'd like to remove it.

Incidentally, this also documents DIST_TRY_BUILD as I fail to find it from a simple search any time I look for it to eg enable tests on try builds.

Author: matthiaskrgr