validate-install-from-source: add workflow dispatch

Add workflow dispatch trigger to more easily run install-from-source
validation workflow.

Author: ldennington

.github/workflows/validate-install-from-source.yml

@@ -1,6 +1,7 @@
 name: validate-install-from-source
 
 on:
+  workflow_dispatch:
   push:
     branches:
       - main