treewide: Fix unsafe concatenation of $LD_LIBRARY_PATH

Naive concatenation of $LD_LIBRARY_PATH can result in an empty
colon-delimited segment; this tells glibc to load libraries from the
current directory, which is definitely wrong, and may be a security
vulnerability if the current directory is untrusted.  (See #67234, for
example.)  Fix this throughout the tree.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

pkgs/servers/plex/default.nix

@@ -97,6 +97,6 @@ buildFHSUserEnv {
 
     # Actually run Plex, prepending LD_LIBRARY_PATH with the libraries from
     # the Plex package.
-    LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$root exec "$root/Plex Media Server"
+    LD_LIBRARY_PATH=$LD_LIBRARY_PATH''${LD_LIBRARY_PATH:+:}$root exec "$root/Plex Media Server"
   '';
 }