Add workflow dispatch to all ci workflows

Author: jhmgoossens

.github/workflows/linux-ci.yml

@@ -1,6 +1,7 @@
 ---
 name: Linux build and test
 on:
+  workflow_dispatch:
   push:
     branches:
       - 'master'

.github/workflows/windows-ci.yml

@@ -1,6 +1,7 @@
 ---
 name: Windows build and test
 on:
+  workflow_dispatch:
   push:
     branches:
       - 'master'