Disabling self-update for vendor installs

Currently packaging boot to be installed via vendor tooling, which gives sysadmin people better peace of mind than a blind `sudo bash curl ...` invocation.

Presently doing this via simply provisioning a custom version of `boot.sh` without all the magic, and deploying the stable boot.jar into `/usr/` and having boot.sh load that. ( It still loads all the dependencies and stuff from maven on first run, but this of course happens as a non-priveleged instead of root ).

However, in such situation its desirable to disable self-update mechanics.

Obviously this can be bodged in via re-writing args passing from scratch in the bash wrapper, and then bailing when somebody tries to upgrade, but I figured it might be better to suggest some kind of first-class support for this.

I suspect a JVM Option like `-Dboot.vendor_install=1` would be sufficient, or maybe a string like `-Dboot.vendor=gentoo` which could trip the right paths in boot.

Start by tracing self-update behavior from boot.sh into boot.jar, then inspect how JVM options are passed through the wrapper. Define how a vendor-install setting should be detected and ensure it prevents self-update without requiring a rewritten wrapper; no specific test file is named in the issue.

Written by the indexing model from the issue text.