externals: Use upstream zlib

We don't need to depend on a custom fork for this. We can add the
library as is, and then make it excluded from the ALL target, so we only
link in the libraries that we actually make use of.

.ci/scripts/linux/docker.sh

@@ -11,4 +11,5 @@ ninja
 
 ccache -s
 
-ctest -VV -C Release
+# Ignore zlib's tests, since they aren't gated behind a CMake option.
+ctest -VV -E "(example|example64)" -C Release