sdk/enter: unconditionally unset GNUGHOME
sdk/enter: unconditionally unset GNUGHOME

Always unset GNUPGHOME since if there is a ~/.gnupg we're bind mounting
then we're always binding it to the default for GNUPGHOME and if there's
not, we don't want to touch GPG at all, so we should not carry over the
environment variable.

glevand/coreos--mantle,dm0-/mantle,glevand/coreos--mantle,dm0-/mantle