dotnet: Avoid double escaping of path

Paths with spaces were escaped within the quotes, resulting in situations
like this one:

```
$ nix-shell maintainers/scripts/update.nix --argstr package nexusmods-app
[omitted]
realpath: "'/home/username/my projects/nixpkgs/pkgs/by-name/ne/nexusmods-app/deps.nix'": No such file or directory
```

Co-authored-by: Matt Sturgeon <matt@sturgeon.me.uk>

pkgs/build-support/dotnet/add-nuget-deps/default.nix

@@ -114,7 +114,8 @@ attrs
 
           # this needs to be before TMPDIR is changed, so the output isn't deleted
           # if it uses mktemp
-          depsFile=$(realpath "''${1:-${lib.escapeShellArg defaultDepsFile}}")
+          ${lib.toShellVars { inherit defaultDepsFile; }}
+          depsFile=$(realpath "''${1:-$defaultDepsFile}")
 
           export TMPDIR
           TMPDIR=$(mktemp -d -t fetch-deps-${lib.escapeShellArg finalPackage.name}.XXXXXX)