chore: use constant format strings

[doringeman]

Fix non-constant format string.

$ go vet ./...
# github.com/docker/model-cli/commands
# [github.com/docker/model-cli/commands]
commands/compose.go:132:15: non-constant format string in call to fmt.Printf
commands/run.go:97:5: non-constant format string in call to fmt.Errorf
commands/run.go:103:22: non-constant format string in call to fmt.Errorf
commands/unload.go:43:6: non-constant format string in call to fmt.Errorf
commands/unload.go:52:5: non-constant format string in call to fmt.Errorf