chore(action): branding colour blue, matching the rest of the project (#38)

purple matched nothing else. Cyan is not in GitHub's fixed palette for action
branding, so blue is the closest to the identity the icon and the README header
already use.

The Marketplace listing renders this colour, so it is worth settling before the
Action is listed rather than after.

Author: dennishavermans

action.yml

@@ -15,7 +15,7 @@ author: Dennis Havermans
 
 branding:
   icon: check-circle
-  color: purple
+  color: blue
 
 inputs:
   command: