GHA: accept extra 'context' input

This is meant for the invoker (ie, msys2-autobuild-controller) to
provide additional information to be logged with the job (specifically,
what user requested it).

.github/workflows/build.yml

@@ -8,6 +8,11 @@ on:
         default: ''
         required: false
         type: string
+      context:
+        description: 'Extra information from invoker'
+        default: ''
+        required: false
+        type: string
   schedule:
     - cron: '0 0/3 * * *'
 

.github/workflows/maint.yml

@@ -13,6 +13,11 @@ on:
         default: ''
         required: false
         type: string
+      context:
+        description: 'Extra information from invoker'
+        default: ''
+        required: false
+        type: string
 
 permissions:
   contents: write