Feature option values are changed before install.sh sees them: quotes are stripped, and $… is expanded or executed #1324

Problem

A Feature can receive a different string option value from the one configured in
devcontainer.json. Quotes are removed, $VAR is expanded, $(…) and backticks are executed, and backslashes are consumed. The build still succeeds, so nothing
signals that the value changed.

Repro (devcontainer CLI 0.89.0)

A local Feature with one string option, value, whose install.sh prints it:

#!/bin/sh
printf '%s\n' "$VALUE"

"features": { "./echo-opt": { "value": "say \"hi\" to $HOME as $(id -un)" } }

• Expected: say "hi" to $HOME as $(id -un)
• Actual: say hi to /root as root

A value with a space inside quotes, such as A="two words", fails the build instead,
with devcontainer-features.env: line 1: words: not found.

Cause

getFeatureEnvVariables (src/spec-node/containerFeatures.ts) writes each option as
NAME="value" without escaping. The install wrapper then sources the file
(set -a; . ./devcontainer-features.env), so the shell interprets the value. Issue #148
and PR #149 fixed this for spaces, but not for ", $, backticks or \.

Suggested fix

Write each value in single quotes, with ' escaped as '\''. That's what
escapeQuotesForShell already does for the wrapper's Options echo. This changes
behaviour for anyone relying on $ being expanded in option values today.