Add powerpc64le-unknown-linux-musl to CI rustc targets

I missed this in the promotion to tier 2 with host tools.

Signed-off-by: Jens Reidel <adrian@travitia.xyz>

Author: Gelbpunkt

src/bootstrap/src/core/download.rs

@@ -875,6 +875,7 @@ pub(crate) fn is_download_ci_available(target_triple: &str, llvm_assertions: boo
         "powerpc-unknown-linux-gnu",
         "powerpc64-unknown-linux-gnu",
         "powerpc64le-unknown-linux-gnu",
+        "powerpc64le-unknown-linux-musl",
         "riscv64gc-unknown-linux-gnu",
         "s390x-unknown-linux-gnu",
         "x86_64-apple-darwin",