Enable other native subsets for linux-riscv64 in CI (#85443)

Follow up on 277a28d (which enabled `linux-riscv64` coreclr build in the CI):

This PR adds `linux-riscv64 Release AllSubsets_Mono` leg in the CI to build `mono`, `libs`, `host` and `packs` subsets.

We are not running tests in the CI, but this leg is to protect the build from regressing.

eng/pipelines/runtime.yml

@@ -393,6 +393,7 @@ extends:
           runtimeFlavor: mono
           platforms:
           - linux_musl_x64
+          - linux_riscv64
           jobParameters:
             testGroup: innerloop
             nameSuffix: AllSubsets_Mono