change_install_name.py was operating on framework library files
in-place, which breaks GN's timestamp analysis handling since a file
can't be both an input and output of an action. As a result no-op builds
on macOS were not actually no-ops.

This changes the script to operate on an output copy, both fixing the
no-op build issue, and simplifying the GN framework construction scripts
by combining the copy step and the install-name step.

Fixes https://github.com/flutter/flutter/issues/33465