Replace isDragonfly with isDragonFly in nix-tools

[hamishmack]

Fixes #2312

[thomasjm]

Hi @hamishmack -- there are a couple other cases of irregular capitalization that need to be handled to fully fix #2312.

I think the following cases need to be added to the fixSystem function:

fixSystem "isHpux" = "isHPUX"
fixSystem "isIos" = "isIOS"
fixSystem "isIrix" = "isIRIX"

I'm looking at the values here:

haskell.nix/lib/cabal-os-arch-comp.nix

Lines 18 to 38 in 70b7857

	os = {
	# OSs
	isLinux = false;
	isWindows = false;
	isOsx = false;
	isIos = false;
	isFreebsd = false;
	isOpenbsd = false;
	isNetbsd = false;
	isDragonFly = false;
	isSolaris = false;
	isAix = false;
	isHPUX = false;
	isIRIX = false;
	isHalvm = false;
	isHurd = false;
	isIOS = false;
	isAndroid = false;
	isGhcjs = false;
	isWasi = false;
	};

I tried to do this myself but it looks like I'd have to deploy a new nix-tools to test it.