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

Turns out it's already installed by default,
but under a different name that's causing extra ops
for no real gain.

.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 pkg-config
+		sudo apt-get install -yq bison libpng-dev
 		;;
 	macos)
 		# macOS bundles GNU Make 3.81, which doesn't support synced output.