Add a way to clean build directories prior to successfully building a package / in general

This is a followup from https://github.com/fosskers/aura/issues/955#issuecomment-3615016691. As part of the linked issue we found that, at least to the best of my knowledge, in the event a PKGBUILD cannot successfully build if invoked in an unclean build directory, as `aura` does on re-runs when not invoked with `--clean`, there is no in-built method to recover for `aura`. Users would have to manually execute some sort of `rm -rf $XDG_CACHE_DIR/aura/builds/<package>` command or similar. It would be useful to have some way to force a build directory cleaning _prior_ to building a package, rather than _after_ building a successful package. Similarly, perhaps the `aura cache` subcommand could be extended to be able to operate upon `cache/aura/builds` as well, for those of us who don't use `--clean` as it seems the `builds` directory just grows without bound with old built packages otherwise currently. ``` $ du -hcs aura/builds/dropbox/* 4.0K aura/builds/dropbox/DropboxGlyph_Blue.svg 112M aura/builds/dropbox/dropbox-lnx.x86_64-205.4.5765.tar.gz 4.0K aura/builds/dropbox/dropbox-lnx.x86_64-205.4.5765.tar.gz.asc 109M aura/builds/dropbox/dropbox-lnx.x86_64-211.4.6008.tar.gz 4.0K aura/builds/dropbox/dropbox-lnx.x86_64-211.4.6008.tar.gz.asc 107M aura/builds/dropbox/dropbox-lnx.x86_64-224.4.4811.tar.gz 4.0K aura/builds/dropbox/dropbox-lnx.x86_64-224.4.4811.tar.gz.asc 69M aura/builds/dropbox/dropbox-lnx.x86_64-234.4.5591.tar.gz 4.0K aura/builds/dropbox/dropbox-lnx.x86_64-234.4.5591.tar.gz.asc 68M aura/builds/dropbox/dropbox-lnx.x86_64-237.4.5655.tar.gz 4.0K aura/builds/dropbox/dropbox-lnx.x86_64-237.4.5655.tar.gz.asc 4.0K aura/builds/dropbox/dropbox.service 4.0K aura/builds/dropbox/dropbox@.service 214M aura/builds/dropbox/pkg 4.0K aura/builds/dropbox/PKGBUILD 887M aura/builds/dropbox/src 24K aura/builds/dropbox/terms.txt 1.6G total ``` At the very least being able to clear out the older final packages would be good housekeeping. The reason I don't use `--clean` by default is to aid in debugging in the event a successfully installed package still has issues, and in the hopes at least some of my installed PKGBUILDs can gain benefits from incremental re-builds (though that mainly would apply to `-git` packages I suppose). Though I see now at least a periodic full clean wouldn't be a bad idea (and one that can currently only be done through manual `rm`).