Move s390x-unknown-linux-musl to build.sh

.github/workflows/bors.yml

@@ -136,8 +136,6 @@ jobs:
           # FIXME: It seems some items in `src/unix/mod.rs`
           # aren't defined on redox actually.
           # x86_64-unknown-redox,
-          # 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       - uses: rust-lang/simpleinfra/github-actions/cancel-outdated-builds@master

ci/build.sh

@@ -238,6 +238,7 @@ riscv32imc-unknown-none-elf \
 riscv32gc-unknown-linux-gnu \
 riscv64gc-unknown-none-elf \
 riscv64imac-unknown-none-elf \
+s390x-unknown-linux-musl \
 sparc-unknown-linux-gnu \
 sparc64-unknown-netbsd \