Revert "Stop trying to explicitly install pkg-config in CI (#1940)"

This reverts commit bc3572ec37.

Turns out that our Dockerfile *does* need to install `pkg-config`
on the `debian:12-slim` image.

.github/scripts/install_deps.sh

@@ -5,7 +5,7 @@ set -eu
 case "${1%-*}" in
 	ubuntu)
 		sudo apt-get -qq update
-		sudo apt-get install -yq bison libpng-dev
+		sudo apt-get install -yq bison libpng-dev pkg-config
 		;;
 	macos)
 		# macOS bundles GNU Make 3.81, which doesn't support synced output.