Replace HOME environment variable access to be more multiplatform

[talister]

Describe the issue
On Windows systems, the HOME environment variable is not set or available and so the

_ROOTDIR = os.path.join(os.environ["HOME"], ".finkclient")

in fink_client\configuration.py will not work. This can be easily replaced by:

_ROOTDIR = os.path.join(os.path.expanduser('~'), ".finkclient")

which is multi-platform and works on Windows, Mac OS X and Linux. (See 2nd part of this SO answer)

System Information:

• Operating System: [Windows 11]:
• Fink client version: [8.7]
• Occurred on which branch and with what commit: [master-9903e97]