Target an even older version of Mac OS X

We can, so let's not ask if we should!

More realistically, we can right now so it's nice to mark that we do, but this is subject to be re-evaluated if any later changes conflicts with this.
Note also that this only affects the binaries we distribute with our releases!
It may very well be possible to compile RGBDS on even older versions of Mac OS X if you have the appropriate compiler setup;
we're just not testing it nor offering such binaries ourselves, but third-party packagers are welcome to do so.
(Looking at you, TigerBrew! ;)

.github/scripts/install_deps.sh

@@ -30,10 +30,18 @@ case "${OS%%-*}" in
 		sudo apt-get install -yq $pkgs
 		;;
 	macos)
+		pkgs=bison
+		case $TOOLSET in
+			'' | lld)
+				pkgs="$pkgs $TOOLSET"
+				TOOLSET=
+			;;
+		esac
 		# macOS bundles GNU Make 3.81, which doesn't support synced output.
 		# We leave it as the default in `PATH`, to test that our Makefile works with it.
 		# However, CMake automatically uses Homebrew's `gmake`, so our CI has synced output.
-		brew install bison make
+		# shellcheck disable=SC2086 # (This word splitting is intentional.)
+		brew install $pkgs make
 		# Export `bison` to allow using the version we install from Homebrew,
 		# instead of the outdated one preinstalled on macOS (which doesn't even support `-Wall`...).
 		export PATH="$(brew --prefix)/opt/bison/bin:$PATH"