update setting keys to deno env

[slim-hmidi]

According to the new version of deno, setting a new key can't be done using : Deno.env.set(key,val) . You will get this error, with the new version of deno:

TS2339: Property 'set' does not exist on type '{ (): { [index: string]: string; }; (key: string): string | undefined; }'.

24      Deno.env.set(key, val);
              ~~~

In my PR, I update the mod.ts file to take into consideration this issue.

[crowlKats]

This is as it was before i updated a few days ago. v0.42.0 changed the behaviour of env to a map-like state (denoland/deno#4942). Most likely you havent updated yet to the latest.

[slim-hmidi]

@crowlKats sorry, so I will update deno.