The self-upgrade feature is undesirable when deno is installed from
(Linux) distribution repository - using a system package manager. This
change will allow package maintainers to build deno with the "upgrade"
subcommand and background check disabled.
When the user runs `deno upgrade <args>` and the upgrade feature is
disabled, it will exit with error message explaining that this deno
binary was built without the upgrade feature.
Note: This patch is already used in the Alpine Linux’s
[deno](https://pkgs.alpinelinux.org/packages?name=deno) package.
