env: rename "system" to "base" in output of env info

[radoering]

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

When running poetry env info, information about the currently active virtualenv and it's base Python is printed. However, the base Python is called system, which might be confusing if the base Python is not the system Python. For example, consider the following sequence of commands:

$ poetry env use 3.9
Using virtualenv: /home/randy/.cache/pypoetry/virtualenvs/poetry-gXMMKm0L-py3.9

$ poetry env info

Virtualenv
Python:         3.9.16
Implementation: CPython
Path:           /home/randy/.cache/pypoetry/virtualenvs/poetry-gXMMKm0L-py3.9
Executable:     /home/randy/.cache/pypoetry/virtualenvs/poetry-gXMMKm0L-py3.9/bin/python
Valid:          True

System
Platform:   linux
OS:         posix
Python:     3.9.16
Path:       /usr
Executable: /usr/bin/python3.9

$ poetry env use system
Deactivating virtualenv: /home/randy/.cache/pypoetry/virtualenvs/poetry-gXMMKm0L-py3.9

$ poetry env info

Virtualenv
Python:         3.8.10
Implementation: CPython
Path:           /home/randy/.cache/pypoetry/virtualenvs/poetry-gXMMKm0L-py3.8
Executable:     /home/randy/.cache/pypoetry/virtualenvs/poetry-gXMMKm0L-py3.8/bin/python
Valid:          True

System
Platform:   linux
OS:         posix
Python:     3.8.10
Path:       /usr
Executable: /usr/bin/python3.8

Thus, it probably makes more sense to call it "base" instead of "system".

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