Disable x86_64-unknown-linux-gnux32 test case (Linux x32 ABI)

This only runs in scheduled jobs. And it doesn't work right now. See #1480 (closed).

It hasn't passed for at least a month, but it's not clear how much of that time it was being run because the osx failure would mask it.

I ran an experiment where I put this job first, and let it run on my branch, and, with this change, it passed: https://gitlab.torproject.org/Diziet/arti/-/pipelines/181071

ETA: NB this is not normal i386 aka x86_32. It's the funky and not-widely-used x32 ABI.