gopass tries to write a version number to it's configuaration, even when
just generating the shell completion scripts. This fails, as
/homeless-shelter is read-only inside the sandbox.
As error messages are printed to stdout instead of stderr
(see https://github.com/gopasspw/gopass/issues/877), the error message
lands inside the completion script, thus breaking it.
Workaround that by setting GOPASS_CONFIG to `/dev/null`
1588d6a549