Add python3.11-venv package to Ubuntu instructions

[JPablomr]

Description

This updates the install instructions for Ubuntu24 to include the python3.11-venv package.

I couldn't get the install to work until I installed this package, I was seeing

Error: Command '['/opt/automatic1111/stable-diffusion-webui/venv/bin/python3.11', '-m', 'ensurepip', '--upgrade', '--default-pip']' returned non-zero exit status 1.

and then it would error out with:

ERROR: Cannot activate python venv, aborting...

Screenshots/videos:

Before:

###############################################################
+ cd /opt/automatic1111/stable-diffusion-webui/..//stable-diffusion-webui/
+ [[ ! -d venv ]]
+ python3.11 -m venv venv
Error: Command '['/opt/automatic1111/stable-diffusion-webui/venv/bin/python3.11', '-m', 'ensurepip', '--upgrade', '--default-pip']' returned non-zero exit status 1.
+ venv/bin/python -m pip install --upgrade pip
/opt/automatic1111/stable-diffusion-webui/venv/bin/python: No module named pip
+ first_launch=1
+ [[ -f venv/bin/activate ]]
+ printf '\n%s\n' '################################################################'

################################################################
+ printf '\e[1m\e[31mERROR: Cannot activate python venv, aborting...\e[0m'
ERROR: Cannot activate python venv, aborting...+ printf '\n%s\n' '################################################################'

After:

+ cd /opt/automatic1111/stable-diffusion-webui/..//stable-diffusion-webui/
+ [[ ! -d venv ]]
+ python3.11 -m venv venv
+ venv/bin/python -m pip install --upgrade pip
Requirement already satisfied: pip in ./venv/lib/python3.11/site-packages (24.0)
Collecting pip
  Using cached pip-24.2-py3-none-any.whl.metadata (3.6 kB)
Using cached pip-24.2-py3-none-any.whl (1.8 MB)
Installing collected packages: pip
  Attempting uninstall: pip
    Found existing installation: pip 24.0
    Uninstalling pip-24.0:
      Successfully uninstalled pip-24.0
Successfully installed pip-24.2
+ first_launch=1
+ [[ -f venv/bin/activate ]]
+ source venv/bin/activate

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• (N/A) My code follows the style guidelines
• (N/A) My code passes tests