helper: set NO_COLOR env for p2pool

The undisplayable ANSI codes appearing in Gupax's
P2Pool console were indeed color codes.

P2Pool's "Log started" message doesn't check `--no-color`,
but it does check `NO_COLOR`, so set that before starting.

src/helper.rs

@@ -491,6 +491,7 @@ impl Helper {
 		debug!("P2Pool | Creating command...");
 		let mut cmd = portable_pty::CommandBuilder::new(path.as_path());
 		cmd.args(args);
+		cmd.env("NO_COLOR", "true");
 		cmd.cwd(path.as_path().parent().unwrap());
 		// 1c. Create child
 		debug!("P2Pool | Creating child...");