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/science/math/arpack/default.nix

@@ -30,9 +30,9 @@ stdenv.mkDerivation {
   ];
 
   preCheck = if stdenv.isDarwin then ''
-    export DYLD_LIBRARY_PATH=$DYLD_LIBRARY_PATH:`pwd`/lib
+    export DYLD_LIBRARY_PATH=$DYLD_LIBRARY_PATH''${DYLD_LIBRARY_PATH:+:}`pwd`/lib
   '' else ''
-    export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:`pwd`/lib
+    export LD_LIBRARY_PATH=$LD_LIBRARY_PATH''${LD_LIBRARY_PATH:+:}`pwd`/lib
   '' + ''
     # Prevent tests from using all cores
     export OMP_NUM_THREADS=2