Specify drainpipe-sass script interpreter

[quicksketch]

Fixes #176

[deviantintegral]

I think the test failures are unrelated to this PR.

[pjudge]

Chiming in to say I ran into this exact issue with adding Drainpipe to a local test site. I employed @quicksketch 's fix and it worked. 👍

[deviantintegral]

Hopefully updating from main fixes the failing job.

@justafish not sure if this could use a test of some kind. If so, perhaps you and @pjudge could collaborate on it?

[justafish]

Thanks!