ci: remove py2-rust support
Nobody cares about this very narrow usecase, and py2 support is over by
July 1st. This helps with the CI load, and removes some flakiness.
## Avoid interference from actual test env:unset HGTEST_JOBSunset HGTEST_TIMEOUTunset HGTEST_PORTunset HGTEST_SHELL