Draft: #2592: Fix k3d-dev reference to hardcoded ubuntu username

General MR

Summary

This removes a hardcoded reference to the ubuntu username in the k3d-dev.sh script and forces it to use the user-provided SSH username instead (which defaults to ubuntu)

Relevant logs/screenshots

Before

quickstart.sh -U myusername
# ... snip ...
+ sudo usermod -aG docker ubuntu
usermod: user 'ubuntu' does not exist

After

quickstart.sh -U myusername
# ... snip ...
+ sudo usermod -aG docker myusername

Linked Issue

Closes #2592 (closed)

Upgrade Notices

N/A