move gpg tmp home to system temp dir

move gpg tmp home to system temp dir

test: Put gpg temp home directory in system temp directory, not filesystem
being tested.

Since I've found indications gpg can fail talking to the agent when the
socket ends up on eg, fat. And to hopefully fix this bug report I've
followed up on.

The main risk in using the system temp dir is that TMPDIR could be set to a
long directory path, which is too long to put a unix socket in. To
partially amelorate that risk, it uses either an absolute or a relative
path, whichever is shorter. (Hopefully gpg will not convert it to a longer
form of the path..)

If the user sets TMPDIR to something so long a path to it +
"S.gpg-agent" is too long, I suppose that's their issue to deal with.

Sponsored-by: Dartmouth College's Datalad project