CI for JSON validation

* CI for JSON validation. Fixes #30.
* Run build CI on new pull-request.

.github/workflows/build.yml

@@ -1,6 +1,6 @@
 name: Build matrix
 
-on: [push, workflow_dispatch]
+on: [push, pull_request, workflow_dispatch]
 
 env:
   OPAMVERBOSE: 1
@@ -63,3 +63,13 @@ jobs:
       run: |
         eval $(opam env)
         etc/ci_core_tests.sh
+
+    - name: Checkout `sail-riscv` repo
+      uses: actions/checkout@v3
+      with:
+        repository: ThinkOpenly/sail-riscv
+
+    - name: Validate Json
+      run: |
+          eval $(opam env)
+          make check-json