.github: remove explicit required from build dispatch

Reduce number of lines. Required = false is the default.

Signed-off-by: Richard Alpe <richard@bit42.se>

.github/workflows/build.yml

@@ -5,17 +5,14 @@ on:
     inputs:
       flavor:
         description: 'Optional build flavor (e.g. _minimal)'
-        required: false
         default: ''
         type: string
       parallel:
         description: 'Massive parallel build of each image'
-        required: false
         default: true
         type: boolean
       infix_repo:
         description: 'Repo to checkout (for spin overrides)'
-        required: false
         default: kernelkit/infix
         type: string