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/applications/science/math/sage/sage-env.nix

@@ -177,7 +177,7 @@ writeTextFile rec {
     export SAGE_EXTCODE='${sagelib.src}/src/ext'
 
   # for find_library
-    export DYLD_LIBRARY_PATH="${lib.makeLibraryPath [stdenv.cc.libc singular]}:$DYLD_LIBRARY_PATH"
+    export DYLD_LIBRARY_PATH="${lib.makeLibraryPath [stdenv.cc.libc singular]}''${DYLD_LIBRARY_PATH:+:}$DYLD_LIBRARY_PATH"
   '';
 } // {
   lib = sagelib; # equivalent of `passthru`, which `writeTextFile` doesn't support