when libsecret is enabled, and no-external-cache is not set for gpg-agent, the
pinenentries will always show the "save in password manager" option.
however, if there is no backend available, then the selection will fail.
It would be preferable if the checkbox only displays when we know that a
password service is actually available.
Unfortunately, doing so probably means using something more sophisticated than
the libsecret "simple API".