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/grpc/default.nix

@@ -38,7 +38,7 @@ stdenv.mkDerivation rec {
   '';
 
   preBuild = ''
-    export LD_LIBRARY_PATH=$(pwd):$LD_LIBRARY_PATH
+    export LD_LIBRARY_PATH=$(pwd)''${LD_LIBRARY_PATH:+:}$LD_LIBRARY_PATH
   '';
 
   NIX_CFLAGS_COMPILE = stdenv.lib.optionalString stdenv.cc.isClang "-Wno-error=unknown-warning-option";