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/development/libraries/caf/default.nix

@@ -22,8 +22,8 @@ stdenv.mkDerivation rec {
   doCheck = true;
   checkTarget = "test";
   preCheck = ''
-    export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$PWD/lib
-    export DYLD_LIBRARY_PATH=$DYLD_LIBRARY_PATH:$PWD/lib
+    export LD_LIBRARY_PATH=$LD_LIBRARY_PATH''${LD_LIBRARY_PATH:+:}$PWD/lib
+    export DYLD_LIBRARY_PATH=$DYLD_LIBRARY_PATH''${DYLD_LIBRARY_PATH:+:}$PWD/lib
   '';
 
   meta = with stdenv.lib; {