ci: exclude arm-linux-musl from native gem builds

Upstream rb-sys added arm-linux-musl to the platform list fetched by CI.
Treat 32-bit ARM Linux as unsupported by the build tools, following the
existing arm-linux exclusion, and exclude the musl variant as well.

Author: ledsun

.github/workflows/build-gems.yml

@@ -25,7 +25,7 @@ jobs:
         id: fetch
         with:
           supported-ruby-platforms: |
-            exclude: [arm-linux, x64-mingw32, aarch64-mingw-ucrt]
+            exclude: [arm-linux, arm-linux-musl, x64-mingw32, aarch64-mingw-ucrt]
           stable-ruby-versions: |
             exclude: [head]