Install dotnet-script in CI for C# script tests

The CSX tests require the dotnet-script global tool to execute
.csx files via `dotnet script`. Without it, all CSX tests fail
with "command or file was not found" on the CI runner.

Author: MelbourneDeveloper

.github/workflows/pr.yml

@@ -47,6 +47,9 @@ jobs:
         with:
           dotnet-version: "10.0.x"
 
+      - name: Install dotnet-script
+        run: dotnet tool install -g dotnet-script
+
       - name: Build
         run: dotnet build --nologo