-share: allow to customize output URL

[dolmen]

For -share, add support for an environment variable that allows to switch the result URL from the default https://go.dev/play/ to something else such as https://goplay.tools/snippet/

Note: we also need to keep room for switch the upload URL for sending to an alternate playground.

Rationale

https://goplay.tools/ is an alternate rendering of snippets posted to https://play.golang.org via https://go.dev: they both share the same storage. But goplay.tools could be prefered by users.
Example:

• https://goplay.tools/snippet/VRAOHHr3xO4
• https://go.dev/play/VRAOHHr3xO4