Make some build-time programs optional

[alexrp]

graf/build.zig

Lines 13 to 16 in 5267fa7

	// TODO: https://github.com/ziglang/zig/issues/15373
	const pandoc_prog = b.findProgram(&.{"pandoc"}, &.{}) catch @panic("Could not locate `pandoc` program.");
	const npm_prog = b.findProgram(&.{"npm"}, &.{}) catch @panic("Could not locate `npm` program.");
	const code_prog = b.findProgram(&.{"code"}, &.{}) catch @panic("Could not locate `code` program.");

graf/.github/workflows/build.yml

Lines 50 to 55 in 4ead4aa

	# TODO: Remove this hack when `code` is made optional.
	- name: Workaround for issue 6
	run: |
	mkdir hack
	touch hack/code
	echo "$PWD/hack" >> $GITHUB_PATH

code, at least, should be optional.

Requires:

• RFC: add the std.Build.fail function and std.Build.FailStep struct ziglang/zig#15373

[alexrp]

Will be done when we update to Zig 0.14.0.

[alexrp]

d4118f5 is working around this until the Zig update.