Allow special characters in flake paths

Support using nix flakes in paths with spaces or abitrary unicode characters.
This introduces the convention that the path part of the URL should be
percent-encoded when dealing with `path:` urls and not when using
filepaths (following the convention of firefox).

Co-authored-by: Rendal <rasmus@rend.al>

src/libutil/url.cc

@@ -103,7 +103,7 @@ std::string percentEncode(std::string_view s, std::string_view keep)
             || keep.find(c) != std::string::npos)
             res += c;
         else
-            res += fmt("%%%02X", (unsigned int) c);
+            res += fmt("%%%02X", c & 0xFF);
     return res;
 }