Add ConTeXt to supported checkformat

[muzimuzhi]

Is ConTeXt officially supported to be set as checkformat? (If not I should revert the change about checkformat.) l3build already has

l3build/l3build-variables.lua

Lines 119 to 124 in 1c63054

	specialformats.context = specialformats.context or {
	luametatex = {binary = "context", format = ""},
	luatex = {binary = "context", format = "", options = "--luatex"},
	pdftex = {binary = "texexec", format = ""},
	xetex = {binary = "texexec", format = "", options = "--xetex"}
	}

The sub-table specialformats.context is contributed by commits e1f620a and 3de5728.