build: pass pip keyring related environment variables

[aucampia]

On Linux, I need to have either DBUS_SESSION_BUS_ADDRESS or some other keyring related environment variable set to avoid keyring related errors. This patch adds these environment variables to the list of environment variables that tox passes through to the test environment.

For the relevant documentation, see:

• DBUS_SESSION_BUS_ADDRESS: https://stackoverflow.com/questions/52264818/runtimeerror-unable-to-initialize-secretservice-environment-variable-dbus-sess
• PIP_KEYRING_PROVIDER and PIP_FORCE_KEYRING: https://pip.pypa.io/en/stable/topics/authentication/
• PYTHON_KEYRING_BACKEND: https://github.com/jaraco/keyring#disabling-keyring

[aucampia]

On my comptuer this is not passing tox still because pyduktape2 is not compiling, looking into it though.

[ashleysommer]

Thanks. I had this same issue last week with a different Poetry project for work, and I fixed it locally with similar environment variables. I didn't realize we had the same issue in this project in the build pipeline. I'll merge this now.