This optimizes test-backend by skipping webhook
tests when run in default mode.
Tweaked by tabbott to extend the documentation and update the CI
commands.
/bin/sh and /usr/bin/env are the only two binaries that NixOS provides
at a fixed path (outside a buildFHSUserEnv sandbox).
This discussion was split from #11004.
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
To support this, we add a pass_targets option to the main linter
library, because with current mypy, it's generally counterproductive
to pass the list of files in (can produce spurious errors; isn't
faster).