Pass through SSH_ env vars to tox envs

This fixes a problem where the tests for fetching a nonexistent ref
prompt like "Enter passphrase for key '/home/USERNAME/.ssh/id_rsa':"
and block, if the repository on the machine where the tests are
being run has the remote set up using an SSH URL.

This passes through all environment variables whose names start
with SSH_, even though it should be enough to pass SSH_AGENT_PID
and SSH_AUTH_SOCK through, at least for this particular issue.

tox.ini

@@ -6,6 +6,7 @@ env_list = py{37,38,39,310,311,312}, lint, mypy, black
 description = Run unit tests
 package = wheel
 extras = test
+pass_env = SSH_*
 commands = pytest --color=yes {posargs}
 
 [testenv:lint]