bors: disable s390x-unknown-linux-musl for now

Author: kaniini

.github/workflows/bors.yml

@@ -134,7 +134,8 @@ jobs:
           # FIXME: It seems some items in `src/unix/mod.rs`
           # aren't defined on redox actually.
           # x86_64-unknown-redox,
-          s390x-unknown-linux-musl,
+          # FIXME: Enable when CI is building a toolchain for this target
+          # s390x-unknown-linux-musl,
         ]
     steps:
       - uses: rust-lang/simpleinfra/github-actions/cancel-outdated-builds@master