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.