chore(ci): add support for manual triggers

see https://github.blog/changelog/2020-07-06-github-actions-manual-triggers-with-workflow_dispatch/

Author: lburgazzoli

.github/workflows/master-snapshot-release.yml

@@ -5,6 +5,7 @@ concurrency:
 on:
   push:
     branches: [ master ]
+  workflow_dispatch:
 jobs:
   test:
     runs-on: ubuntu-latest

.github/workflows/pr.yml

@@ -5,6 +5,7 @@ concurrency:
 on:
   pull_request:
     branches: [ master ]
+  workflow_dispatch:
 jobs:
   build:
     runs-on: ubuntu-latest