Fix FORCE_COLOR to set exact level instead of minimum

Previously, FORCE_COLOR was used as a floor value and terminal
detection could return a higher level. For example, FORCE_COLOR=1
on a truecolor terminal would still give level 3.

Now when FORCE_COLOR is set to a specific level (1, 2, or 3), that
exact level is returned without further terminal capability checks.

Fixes #624

source/vendor/supports-color/index.js

@@ -91,7 +91,15 @@ function _supportsColor(haveStream, {streamIsTTY, sniffFlags = true} = {}) {
 		return 0;
 	}
 
-	const min = forceColor || 0;
+	// When `FORCE_COLOR` is set to a specific level, return that level
+	// directly instead of using it as a minimum. This ensures that
+	// e.g. FORCE_COLOR=1 gives exactly level 1, not a higher level
+	// based on terminal capabilities.
+	if (forceColor !== undefined) {
+		return forceColor;
+	}
+
+	const min = 0;
 
 	if (env.TERM === 'dumb') {
 		return min;