Use util.url.join for URLs in GNU Mirrors / reorder Mirrors (#14395)
Use util.url.join for URLs in GNU Mirrors / reorder Mirrors (#14395)

* Reorder GNU mirrors (#14395)

As @adamjstewart commented in #14395, GNU suggests to use
their mirror. So reorder the mirror to the top.

GNU Doc: https://www.gnu.org/prep/ftp.en.html

* Use spack.util.url.join for URLs in GNU mirrors (#14395)

One should not use os.path.join for URLs. This does only
work on POSIX systems.

Instead use spack.util.url.join.
So every part in spack uses the same url joining method.

LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack