fix-of-a-fix (knative#2994)

Author: gauron99

pkg/builders/buildpacks/builder.go

@@ -217,7 +217,14 @@ func (b *Builder) Build(ctx context.Context, f fn.Function, platforms []fn.Platf
 
 		if f.Runtime == "python" {
 			if fi, _ := os.Lstat(filepath.Join(f.Root, "Procfile")); fi == nil {
-				cli = pyScaffoldInjector{cli, f.Invoke}
+				// HACK (of a hack): need to get the right invocation signature
+				// the standard scaffolding does this in toSignature() func.
+				// we know we have python here.
+				invoke := f.Invoke
+				if invoke == "" {
+					invoke = "http"
+				}
+				cli = pyScaffoldInjector{cli, invoke}
 			}
 		}