Updated code to fix dist issue with env in settings file

[sandeep-vedam]

Following are the scenarios that are verified using the fix:

I have verified this with below cases :

With no esEnv setting in setings.json file
lng dist : Creates dist version of es6
lng dist --es6 : Creates dist version of es6
lng dist --es5: Created dist version of es5
lng dist --es5 --es6 : Creates both es5 and es6

With esEnv setting in setings.json file
lng dist & esEnv:es5 : Creates dist version of es5
lng dist & esEnv:es6 : Creates dist version of es5
lng dist --<es5/es6> & esEnv:es5 : Creates dist version of es5/es6 and ignores the settings.json file esEnv

With different settings file(for ex: settings.es5.json)
lng dist & esEnv:es5 : Creates dist version of es5
lng dist & esEnv:es6 : Creates dist version of es5
lng dist --<es5/es6> & esEnv:es5 : Creates dist version of es5/es6 and ignores the settings.json file esEnv

[michielvandergeest]

LGTM

@cidevant would appreciate if you can take this change for a spin and see if it's fixed fo you as well 😄