"zero install" should live up to its name

[quintesse]

Right now when you do curl sh.jbang.dev | bash -s - you're basically performing a regular installation, the only difference being that no changes are being made to your environment (eg. PATH).

This is perhaps efficient because you don't have to download Jbang on each invocation, but it also means that we have a conflict when an older version Jbang is already installed. We'd either have to overwrite/update the version automatically, which the user might not want/expect, or we'd use the existing version, when the user perhaps expects access to the latest features.

I would suggest that we download to a temporary location that would be automatically cleaned up when Jbang exits. It's less efficient but it will make sure that we always run the latest version without any conflicts with installed Jbang versions.

[quintesse]

PR was reverted. Reopening.