Fix charlist warning for Elixir 1.17

[jgelens]

Prevents the following warning:

     warning: single-quoted strings represent charlists. Use ~c"" if you indeed want a charlist or use "" instead
     │
 348 │         {'location', download} = List.keyfind(headers, 'location', 0)
     │          ~
     │
     └─ lib/dart_sass.ex:348:10

     warning: single-quoted strings represent charlists. Use ~c"" if you indeed want a charlist or use "" instead
     │
 348 │         {'location', download} = List.keyfind(headers, 'location', 0)
     │                                                        ~
     │
     └─ lib/dart_sass.ex:348:56

[mcrumm]

Thanks!