Use os.environ for PROGRAMDATA Path

This fixes #16 with PR #17.

Description

"/c/ProgramData/Multipass/data/ssh-keys/id_rsa" vs "/c/Windows/ProgramData/Multipass/data/ssh-keys/id_rsa"

Suggestions

Windows 11 config file

{
	"host": "dev.multipass.local",
        "hostname": {
            "source": "multipass",
            "name": "dev"
        },
        "user": "ubuntu",
        "identity_file": "/c/ProgramData/Multipass/data/ssh-keys/id_rsa"
}

Windows 10 config file

{
	"host": "dev.multipass.local",
        "hostname": {
            "source": "multipass",
            "name": "dev"
        },
        "user": "ubuntu",
        "identity_file": "/c/Windows/ProgramData/Multipass/data/ssh-keys/id_rsa"
}

https://github.com/iancleary/local-ssh-config/blob/b92b15c5d57b723ee51b038ebae8e710ac479fe6/local_ssh_config/ssh/_constants.py#L9-L11

The final code uses the os.environ variable with a default value: f7feb88#diff-574e09d12929e1211b1e6580c613b5f65f3448c0f6c007348a4aa665459c9a83L10-R14