[Regressions] Support more complex scrubbers (#7819)

[Regressions] Support more complex scrubbers (#7819)

Currently, we only support single commands in our `SCRUBBER` directives
for regression tests. This commit adds support for executing more
complex commands, including pipes. It also updates the regression script
to use the more modern, higher-level `subprocess.run()` instead of
`subprocess.Popen()`.