CI: enable RISC-V for 4.x branch

Author: mshabunin

.github/workflows/PR-4.x.yaml

@@ -26,3 +26,6 @@ jobs:
 
   macOS-X64:
     uses: opencv/ci-gha-workflow/.github/workflows/OCV-Contrib-PR-4.x-macOS-x86_64.yaml@main
+
+  Linux-RISC-V-Clang:
+    uses: opencv/ci-gha-workflow/.github/workflows/OCV-Contrib-PR-4.x-RISCV.yaml