[BUG] PREFIX environment variable ignored by git config --system

[<rsbecker@nexbridge.com>]

Thank you for filling out a Git bug report!

Please answer the following questions to help us understand your issue.

What did you do before the bug happened? (Steps to reproduce your issue)
export PREFIX=/home/randall
git config --system --list

What did you expect to happen? (Expected behavior)
Git should use ${PREFIX}/etc/gitconfig instead of the build location to
locate
System configuration items, per 10.8 in the git-scm website documentation
on Environment Variables.
Git should have reported:
fatal: unable to read config file '/home/randall/etc/gitconfig': No such
file or directory

What happened instead? (Actual behavior)
git used /usr/local-ssl3.0/etc/gitconfig instead and reported that the file
does not exist:
fatal: unable to read config file '/usr/local-ssl3.0/etc/gitconfig': No such
file or directory

What's different between what you expected and what actually happened?
The directory being used based on the PREFIX variable.

Anything else you want to add:
This appears to be a problem on all platforms.

[System Info]
git version:
git version 2.47.1
cpu: NSV
built from commit: 92999a42db1c5f43f330e4f2bca4026b5b81576f
sizeof-long: 4

[System Info]
git version:
git version 2.45.1
cpu: x86_64
no commit associated with this build
sizeof-long: 8

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