uscan: Allow compression of VCS exports to be disabled

In case files need to be excluded from the upstream source, uscan builds a compressed tarball for the VCS export and then mk-origtargz builds a new compressed tarball with the exclusions. This nearly doubles the time taken for large repositories that have exclusions, such as those for linux and firmware-nonfree.

Since there is a trade-off between space and time here, add an option (--vcs-export-uncompressed) and environment variable to disable compression of the exported tarball. In this case, we always tell mk-origtargz to repack it.

Also update a comment in mk-origtargz to note that uncompressed tarballs are supported.

Closes: #1003251