Pylint: enforce C-style string formatting

[reidpr]

Split from #1860. Pylint: C0209 consider-using-f-string

charliecloud/lib/build_cache.py

Lines 178 to 179 in d679373

	ch.FATAL("insufficient Git for build cache mode: %s"
	% cli.bucache.value)

I don’t like f-strings and I’ve not seen any good argument why they are better. Thus, let’s extend Pylint to enforce C-style string formatting instead.