例えば友人に「ロングウィークエンドうれしいね」などと言われて、私にはバイトがあるから「でも私には関係ないんだよねー」とかっていうのは英語でどう表現したらいいんでしょう？？
not for me!　とか？

It doesn’t matter to me.
It has nothing to do with me.
とかじゃないですかね？ 　

not for meでもいいし、
Well, it doesn’t apply to me because I will be working those days.
・・・のような言い回しもありで。 　

Too bad ! It doesn’t mean anything to me. 　

"not for me" is fine.
You could make it better by saying "not for me, I’m working"
　

Or maybe try saying, "yeah, but unfortunately i am stuck working" or something like that.

if you just say "not for me" its kind of a downer for the person talking to you who doesn’t have to work.

　

not my concern はどうですか？
(しったこっちゃない、構うもんか）