make sure that we always untar aur packages to a predictable directory

[jdodds]

I ran into a package whose top-level directory in the tar wasn't the same as the name of the package. This ensures that we'll always be untarring into the "correct" directory.

[bruenig]

Not totally clear what that does, but it looks good, and so I put it in.

[jdodds]

-C dir untars to dir, and --strip-components=1 skips creating the first directory from the tarfile, kinda like patch -p1