tests.dotnet.project-references: use dir instead of file output

This fixes an error in nixpkgs-review when it tries to pass this to
buildEnv:

error: The store path [...]project-references-test is a file and can't
be merged into an environment using pkgs.buildEnv! at [...]-builder.pl
line 122.

Changed files
+1 -1
pkgs
test
dotnet
project-references
+1 -1
pkgs/test/dotnet/project-references/default.nix
···
runCommand "project-references-test" { } ''
${application}/bin/Application
-
touch $out
+
mkdir $out
''