This also fixes a few test installers:
- rustfmt is available via apt-get in 22.04, use that instead of tarball
that's no longer available
- brittany now installs cleanly in the default haskell ghc/cabal in
22.04, so just use that
- perltidy in 22.04 has slightly different output
- nixfmt download url changed (now includes OS/arch). There was
initially no binary available when nixfmt was updated,
but the devs there were kind enough to provide one:
https://github.com/serokell/nixfmt/issues/139
This runs a full `make fmt-build` without errors, though it takes a
while ;). All tests pass, too.
