The shellcheck SC3028 check warns when variables common in sh/bash
are not defined in POSIX sh. This is generally $UID and $EUID.

This check is currently suppressed at repository level, but that prevents
to check other variables like $HOSTNAME, $MACHTYPE or $SHELLOPTS.

Align code of operations repository with recommended D3679 snippet:

• SC3028 is defined at instruction level, not arc configuration
• all scripts use the snippet testing for EUID, with id as fallback

For reference, csh and tcsh are the two shells not implementing UID and EUID.

arc lint