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/guile-modules/guile-lib/default.nix

@@ -21,7 +21,7 @@ in stdenv.mkDerivation {
   preCheck = ''
     # Make `libgcc_s.so' visible for `pthread_cancel'.
     export LD_LIBRARY_PATH=\
-    "$(dirname $(echo ${stdenv.cc.cc.lib}/lib*/libgcc_s.so)):$LD_LIBRARY_PATH"
+    "$(dirname $(echo ${stdenv.cc.cc.lib}/lib*/libgcc_s.so))''${LD_LIBRARY_PATH:+:}$LD_LIBRARY_PATH"
   '';
 
   meta = with stdenv.lib; {