Fails with gpg 2. Instead, use a different environment variable.
The clean fix would instead be to add an annex.gpg-options configuration.
But, that would be rather a lot of work and it's unlikely it would be
useful for much else.
Only set --no-tty when GPG_AGENT_INFO is set and batch mode is used.
In the test suite, set GPG_AGENT_INFO to /dev/null to avoid the test suite
relying on /dev/tty.
