Set a default value for build-event flag of cli exec command (#1212)

Otherwise `cli exec` does not do anything due to the default constraints
applied to a pipeline (i.e., some kind of build event is expected).

cli/exec/flags.go

@@ -157,6 +157,7 @@ var flags = []cli.Flag{
 	&cli.StringFlag{
 		EnvVars: []string{"CI_BUILD_EVENT"},
 		Name:    "build-event",
+		Value:   "manual",
 	},
 	&cli.StringFlag{
 		EnvVars: []string{"CI_BUILD_LINK"},