Summary

Fixes #4694.

This makes the RVM load path tolerate Bash set -u/nounset when no system metadata has been detected yet. The failing Packer/AWS scenario runs RVM from a strict-mode shell, so reads of variables such as _system_name, rvm_prefix, and chpwd_functions should not assume those names have already been initialized.

Changes:

• Guard rvm_prefix before deriving it from rvm_path.
• Fall back to the generic utility setup before _system_name has been detected.
• Initialize and append to chpwd_functions without triggering nounset on Bash 3.2.
• Add a fast regression test for sourcing RVM and selecting the system Ruby under nounset.

Validation

• bash -n scripts/rvm scripts/functions/support scripts/cd tests/fast/nounset_comment_test.sh
• bash -lc 'set -u; export rvm_path=$PWD; source scripts/rvm; rvm use system >/tmp/rvm-use-system.out; cat /tmp/rvm-use-system.out; declare -p chpwd_functions; [[ ":${SHELLOPTS:-}:" == *":nounset:"* ]]'
• bash -lc 'set -u; export rvm_path=$PWD; source scripts/rvm; rvm info system >/tmp/rvm-info-system.out; head -n 22 /tmp/rvm-info-system.out; [[ ":${SHELLOPTS:-}:" == *":nounset:"* ]]'
• git diff --check

Note: the local rvm info system run on macOS emits existing environment warnings about /opt/homebrew/bin permissions and Command Line Tools vs full Xcode, but exits successfully.