revert branch default handling

plugin/impl.go

@@ -46,12 +46,6 @@ func (p *Plugin) run(ctx context.Context) error {
 func (p *Plugin) Validate() error {
 	var err error
 
-	// This default cannot be set in the cli flag, as the CI_* environment variables
-	// can be set empty, resulting in an empty default value.
-	if p.Settings.Repo.Branch == "" {
-		p.Settings.Repo.Branch = "main"
-	}
-
 	if p.Settings.Repo.WorkDir == "" {
 		p.Settings.Repo.WorkDir, err = os.Getwd()
 	}

plugin/plugin.go

@@ -141,6 +141,7 @@ func Flags(settings *Settings, category string) []cli.Flag {
 			Usage:       "name of the git source branch",
 			EnvVars:     []string{"PLUGIN_BRANCH"},
 			Destination: &settings.Repo.Branch,
+			Value:       "main",
 			Category:    category,
 		},
 		&cli.StringFlag{