dist: Add libbacktrace to source tarballs
mk: distcheck --uninstall
mk: Workaround distcheck failure on mac. #13224

Mac can't actually build our source tarballs because it's `tar`
command doesn't support the --exclude-vcs flag. This is just
a workaround to make our mac nightlies work (we get our source
tarballs from the linux bot).
auto merge of #13223 : brson/rust/dist, r=alexcrichton