fix: update fallback user in remote_user_run function to use current user

[darthrevan030]

Fix: fall back to current user when UID 1000 does not exist

Problem:

When using this feature with standard Microsoft devcontainer base images (e.g. mcr.microsoft.com/devcontainers/typescript-node, python, go, cpp), the build fails with:
su: user vscode does not exist or the user entry does not contain all the required fields.

This happens because devcontainer features run during the Docker build phase, but the vscode user (UID 1000) is only created at container startup. So id -un 1000 fails and falls back to the hardcoded string "vscode", which doesn't exist in /etc/passwd at build time.

Fix

In util.sh, change the fallback in remote_user_run from:
sh_REMOTE_USER="$(id -un 1000 2>/dev/null || echo "vscode")" # vscode fallback
to:
sh_REMOTE_USER="$(id -un 1000 2>/dev/null || id -un)" # fallback to current user

id -un returns whoever is actually executing the script (root during build), which is guaranteed to exist. Claude Code's install script handles installing to the correct home directory regardless.