abuild-sudo: don't allow --keys-dir

Not allowing --allow-untrusted is obviously a good idea, but it can be
trivially bypassed if --keys-dir is allowed:

$ abuild-apk add foo-1-r0.apk
ERROR: foo-1-r0.apk: UNTRUSTED signature
$ abuild-apk --allow-untrusted add foo-1-r0.apk
abuild-apk: --allow-untrusted: not allowed option
$ cp -rp /etc/apk/keys /tmp/keys
$ cp untrusted.pub /tmp/keys
$ abuild-apk --keys-dir /tmp/keys add foo-1-r0.apk
(1/1) Installing foo (1-r0)
OK: 4319 MiB in 806 packages

If both --allow-untrusted and --keys-dir are not allowed, then it should
no longer be possible for an unprivileged member of the abuild group to
add an untrusted package.

$ abuild-apk --keys-dir /tmp/keys add foo-1-r0.apk
abuild-apk: --keys-dir: not allowed option