Add a manual way to trigger GitHub Actions workflows

.github/workflows/check_repos.yml

@@ -9,6 +9,7 @@ name: Test official instructions for installing Dangerzone
 on:
   schedule:
     - cron: '0 0 * * *' # Run every day at 00:00 UTC.
+  workflow_dispatch:
 
 jobs:
   install-from-apt-repo:

.github/workflows/ci.yml

@@ -5,6 +5,7 @@ on:
     branches: [ main ]
   schedule:
     - cron: '0 0 * * *' # Run every day at 00:00 UTC.
+  workflow_dispatch:
 
 jobs:
   windows:

.github/workflows/scan.yml

@@ -5,6 +5,7 @@ on:
     branches: [ main ]
   schedule:
     - cron: '0 0 * * *' # Run every day at 00:00 UTC.
+  workflow_dispatch:
 
 jobs:
   security-scan-container:

.github/workflows/scan_released.yml

@@ -2,6 +2,7 @@ name: Scan released app and container
 on:
   schedule:
     - cron: '0 0 * * *' # Run every day at 00:00 UTC.
+  workflow_dispatch:
 
 jobs:
   security-scan-container: