Add manual way to show the size of virtual environments in the PR

[basfroman]

Sometimes when the dependency list changes it will be useful to know how the python virtual environment has changed. Now we can use the show-venv-size label for this.

[github-actions]

Bittensor SDK virtual environment sizes by Python version:

Python 3.9: 195 MB
Python 3.10: 195 MB
Python 3.11: 211 MB
Python 3.12: 205 MB
Python 3.13: 168 MB