libutil: fix double-encoding of URLs

If you have a URL that needs to be percent-encoded, such as
`http://localhost:8181/test/+3d.tar.gz`, and try to lock that in a Nix
flake such as the following:

    {
      inputs.test = { url = "http://localhost:8181/test/+3d.tar.gz"; flake = false; };
      outputs = { test, ... }: {
        t = builtins.readFile test;
      };
    }

running `nix flake metadata` shows that the input URL has been
incorrectly double-encoded (despite the flake.lock being correctly
encoded only once):

    [...snip...]
    Inputs:
    └───test: http://localhost:8181/test/%252B3d.tar.gz?narHash=sha256-EFUdrtf6Rn0LWIJufrmg8q99aT3jGfLvd1//zaJEufY%3D

(Notice the `%252B`? That's just `%2B` but percent-encoded again)

With this patch, the double-encoding is gone; running `nix flake
metadata` will show the proper URL:

    [...snip...]
    Inputs:
    └───test: http://localhost:8181/test/%2B3d.tar.gz?narHash=sha256-EFUdrtf6Rn0LWIJufrmg8q99aT3jGfLvd1//zaJEufY%3D

---

As far as I can tell, this happens because Nix already percent-encodes
the URL and stores this as the value of `inputs.asdf.url`.

However, when Nix later tries to read this out of the eval state as a
string (via `getStrAttr`), it has to run it through `parseURL` again to
get the `ParsedURL` structure.

Now, this itself isn't a problem -- the true problem arises when using
`ParsedURL::to_string` later, which then _re-escapes the path_. It is
at this point that what would have been `%2B` (`+`) becomes `%252B`
(`%2B`).

src/libutil/url.cc

@@ -44,7 +44,7 @@ ParsedURL parseURL(const std::string & url)
             .base = base,
             .scheme = scheme,
             .authority = authority,
-            .path = path,
+            .path = percentDecode(path),
             .query = decodeQuery(query),
             .fragment = percentDecode(std::string(fragment))
         };