document CABAL_CONFIG env variable.

[gbaz]

nix-shell works by setting a bunch of env variables. It would be nice if among those variables it could set would be one that directed cabal to use a config file other than in ~/.cabal/config. That way, we could tweak all of cabal's other settings at once to make it play nicely in the nix environment. This is potentially useful in other settings as well when some other tool wants to put cabal into a specific environment.

[sebright]

CABAL_CONFIG overrides the config file location, though it looks like it's not documented:

cabal/cabal-install/Distribution/Client/Config.hs

Lines 658 to 660 in 3c339d2

	-- | Returns the config file path, without checking that the file exists.
	-- The order of precedence is: input flag, CABAL_CONFIG, default location.
	getConfigFilePath :: Flag FilePath -> IO FilePath

[hvr]

r elatedly, it may be useful to have such an env-var for setting the --project-file value via env as well

[gbaz]

Great! cc @ElvishJerricco. Will leave this ticket open to record documenting it