Temporarily disable powerpc-unknown-linux-gnu tests

As mentioned in [1], this test has started to fail for unclear reasons.
Disable this until it can be investigated further.

[1]: #4254 (comment)

(backport <#4254>)
(cherry picked from commit eb7045b)

Author: tgross35

.github/workflows/ci.yaml

@@ -136,7 +136,9 @@ jobs:
           - i686-unknown-linux-musl
           - loongarch64-unknown-linux-gnu
           - loongarch64-unknown-linux-musl
-          - powerpc-unknown-linux-gnu
+          # FIXME(ppc): SIGILL running tests, see
+          # https://github.com/rust-lang/libc/pull/4254#issuecomment-2636288713
+          # - powerpc-unknown-linux-gnu
           - powerpc64-unknown-linux-gnu
           - powerpc64le-unknown-linux-gnu
           - riscv64gc-unknown-linux-gnu