ci: replace mips with powerpc64, aarch64 and s390x

We drop our MIPS target because it no longer works.[1] We were
previously using it as a means of testing ripgrep in a big endian
environment. So to achieve that without MIPS, we test on powerpc64 and
s390x. (No particular reason to do both, but why not.)

We also add aarch64 as a proxy for at least ensuring everything works
for the same architecture as Apple silicon. It's not a guarantee that
everything works, but it seems better than nothing until we can actually
test Apple silicon in CI.

[1]: c788378d6f

tests/feature.rs

@@ -791,6 +791,9 @@ rgtest!(f1466_no_ignore_files, |dir: Dir, mut cmd: TestCommand| {
 rgtest!(f2361_sort_nested_files, |dir: Dir, mut cmd: TestCommand| {
     use std::{thread::sleep, time::Duration};
 
+    if crate::util::is_cross() {
+        return;
+    }
     dir.create("foo", "1");
     sleep(Duration::from_millis(100));
     dir.create_dir("dir");