packages/pkgbuild: touch PKGBUILD when it is fetched from AUR.

This is needed to rebuild updated packages.

An updated PKGBUILD will have mtime of when maintainer has last modified
it, which is not necessarily newer than previous version's install time
(consider e. g. v1 released - v2 released - v1 installed),
so just re-fetching PKGBUILD may fail to cause a proper package rebuild.

To work around, touch the PKGBUILD when it is updated so the package
will always be rebuilt after it has been updated (which is the desired
behavior, actually).