Use constant format strings with Printf-like functions (#1013)

Recent versions of Go object to the use of non-constant variables a
format strings. This commit fixes errors like this:

cli.go:26:47: non-constant format string in call to fmt.Fprintf

Signed-off-by: W. Michael Petullo <mike@flyn.org>

internal/cli/cli.go

@@ -22,7 +22,7 @@ type Program struct {
 }
 
 func (p *Program) Execute() {
-	flag.Usage = func() { fmt.Fprintf(os.Stderr, p.Usage) }
+	flag.Usage = func() { fmt.Fprint(os.Stderr, p.Usage) }
 	flag.Parse()
 	os.Exit(p.main(flag.Args(), os.Stdin, os.Stdout, os.Stderr))
 }