Please add ability to communicate changed files list via filesystem

[jepler]

If the length of the changed files is long, it can exceed the limit on environment variables (131071 bytes for a single environment variable NAME=value setting on Linux; likely much less on Windows).

Please add the ability to communicate the list of changed files via the filesystem instead, to avoid this limitation.

Related:

• adafruit/circuitpython#5359 (in which the changed files list is also incorrect due to something about my fork that we haven't fixed yet)