Fix checkpkg for other architectures

When building for an architecture different from the host, the correct old package was downloaded as "$copydir"'s pacman was configured with the target CARCH, but checkpkg doesn't know this and tries to search the cache for host CARCH instead, producing the following error:

==> ERROR: tarball not found for package: xxx

This change fixes this by passing MAKEPKG_CONF explicitly, so that checkpkg behaves consistently.