Fix linter warnings

internal/ui/signals/signals.go

@@ -8,7 +8,7 @@ import (
 // GetProgressChannel returns a channel with which a single listener
 // receives each incoming signal.
 func GetProgressChannel() <-chan os.Signal {
-	signals.Once.Do(func() {
+	signals.once.Do(func() {
 		signals.ch = make(chan os.Signal, 1)
 		setupSignals()
 	})
@@ -19,6 +19,6 @@ func GetProgressChannel() <-chan os.Signal {
 // XXX The fact that signals is a single global variable means that only one
 // listener receives each incoming signal.
 var signals struct {
-	ch chan os.Signal
-	sync.Once
+	ch   chan os.Signal
+	once sync.Once
 }