As we do in other places stop using the `actions` runner and use the
`dtolnay` one to checkout toolchain.
While we are at it, use double quotes for `name` fields (this is a small
stylistic thing I have been introducing in an effort to make the yaml
files a bit easier to read).
MIPS was recently downgraded to Tier 3, which means it won't be installable by
rustup and may not work as expected. This commit removes all MIPS-related
CI jobs.
Tobin C. Harding
Martin Habovstiak
Davidson Souza
Riccardo Casatta