pkgbase: rename PKGSIGNKEY to PKG_REPO_SIGNING_KEY

this way, it'll be automatically picked up by poudriere
That's quite handy when building pkgbase!

Submitted by: Mina Galić <me@igalic.co>
Reviewed By: bapt, bcr (manpages)
Differential Revision: https://reviews.freebsd.org/D27690