Linux standalone: Use md5sum to shorten paths in .cache/git-annex/locales

Linux standalone: Use md5sum to shorten paths in .cache/git-annex/locales

md5sum is part of busybox, so is probably available unless it were compiled
out. If md5sum (or cut for that matter) is not available, it will
still use the whole path to $base, otherwise hash it.

Of course it's possible for md5sum to be available sometimes and not others
on the same system; in such an event the locales would be built twice for
the same bundle. The cleanup code will delete both sets once that
version of the bundle is upgraded.