keccak: replace CI tests on MIPS with PPC32 (#62)

`mips-unknown-linux-gnu` is now a Tier 3 target: rust-lang/rust#115218.

This means we can't use it for cross tests anymore since std is no
longer built for it.

This commit replaces it with `powerpc-unknown-linux-gnu`, a big endian
Tier 2 target.

.github/workflows/keccak.yml

@@ -92,7 +92,7 @@ jobs:
           - i686-unknown-linux-gnu
           - x86_64-unknown-linux-gnu
           - aarch64-unknown-linux-gnu
-          - mips-unknown-linux-gnu
+          - powerpc-unknown-linux-gnu
         features:
           - no_unroll
           - 'NO_FEATURE'