gpg: Avoid error diagnostics with --override-session-key when verifying

* src/engine-gpg.c (gpg_decrypt): only send --no-keyring when we are
not verifying.

Without this change, the signature verification would fail. This
problem was introduced in bded8ebc59c7fdad2617f4c9232a58047656834c in
an attempt to avoid an error when *not* verifying. Clearly more test
suite coverage is needed to avoid introducing this sort of problem in
the future.

• GnuPG-bug-id: T3464
• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>