Flutter Failed to build iOS app Error (Xcode): unable to spawn process '/bin/sh' (Argument list too long)

Ok: So go to your Xcode then select File at the top and move to Workspace setting. 
A pop up will appear for you to update your workspace. 
If you have an updated Xcode go to advanced option and select legacy and press done.
Happy Coding :)

출처 : unable to spawn process '/bin/sh' (Argument list too long) (in target 'Runner' from project 'Runner')

My solution is to change the Derived Data folder to a directory with a shorter path.
The Steps is as the following:
Xcode -> File -> Workspace Settings... -> Select Custom Location for Derived Data and give a shorter path as the location. 

출처 : error: unable to spawn process (Argument list too long) in Xcode Build