tidy global variables

[reidpr]

Split from #1860. Pylint:

• C0103 invalid-name
• W0603 global-statement

charliecloud/lib/build_cache.py

Lines 140 to 144 in d679373

	# Absolute path of Git binary we’re using.
	git = None
	# Default path within image to metadata pickle.
	PICKLE_PATH = fs.Path("ch/git.pickle")

Python does not have constants, and the convention Pylint wants is that module globals are treated as constants and use UPPER_CASE naming style. I believe what we’re doing is:

1. Module globals set during import are treated as constants and use UPPER_CASE.

2. Module globals set once, then treated as constant; we use snake_case for these.

3. True global variables, snake_case.

Groups 1 and 2 should be set once only and constant after that. I don't know how many 3 we have.

Pylint also just doesn’t want us to use global at all which honestly makes sense.