Github: Add workflow dispatch trigger for CI builds

.github/workflows/ci-build.yml

@@ -5,6 +5,11 @@ on:
   push:
     branches:
     - master
+  workflow_dispatch:
+    inputs:
+      ref:
+        description: 'Ref to build (for Pull Requests, use refs/pull/NNN/head)'
+        required: true
 
 env:
   CTEST_OUTPUT_ON_FAILURE: 1
@@ -20,8 +25,15 @@ jobs:
 
     steps:
     - name: Checkout
+      if: github.event_name != 'workflow_dispatch'
       uses: actions/checkout@v2
 
+    - name: Checkout (Manual)
+      if: github.event_name == 'workflow_dispatch'
+      uses: actions/checkout@v2
+      with:
+        ref: ${{ github.event.inputs.ref }}
+
     - name: Setup cache
       uses: actions/cache@v2
       with: