buildDotnetModule: unset TMPDIR instead of setting it empty

This was breaking nix-prefetch-url when running fetch-deps in nix-shell.

e.g.

    $ TMPDIR= nix-prefetch-url foo
    nix-prefetch-url: src/libutil/util.cc:119: nix::Path
    nix::canonPath(PathView, bool): Assertion `path != ""' failed. [2]
    881198 abort (core dumped)

pkgs/build-support/dotnet/build-dotnet-module/default.nix

@@ -214,7 +214,7 @@ stdenvNoCC.mkDerivation (args // {
         if [[ ''${TMPDIR:-} == /run/user/* ]]; then
            # /run/user is usually a tmpfs in RAM, which may be too small
            # to store all downloaded dotnet packages
-           TMPDIR=
+           unset TMPDIR
         fi
 
         export tmp=$(mktemp -td "deps-${pname}-XXXXXX")