.github: add mips-32 test

[gballet]

This is an attempt at replacing the Windows 32 appveyor build with something else. mips32 is a prime target as some rollups compile geth to mips in order to build their proofs.

The current version only runs the core package. For it to work, devops needs to prepare the self-hosted machine, which should be done next week.

[fjl]

I closed because we ended up deploying the 32bit tests in another way.