Deal more appropriately with NO_COVERAGE: turn it into a documented input

[wilfwilson]

In practice, compiled packages are always compiled with --coverage flags, because the environment variable NO_COVERAGE is unlikely to have been set by a user, even if that's what they want:

build-pkg/action.yml

Lines 21 to 25 in 3246f13

	if [[ -z $NO_COVERAGE ]]; then
	export CFLAGS="$CFLAGS --coverage"
	export CXXFLAGS="$CXXFLAGS --coverage"
	export LDFLAGS="$LDFLAGS --coverage"
	fi

Therefore, NO_COVERAGE should be made into an input to this Action, as it is in https://github.com/gap-actions/setup-gap. It should also be clearly documented.