fix(git): set HTTP(S)_PROXY only in process scope to prevent race condition

[r15ch13]

Description

Prevents leaking HTTP(S)_PROXY env vars to current sessions after Invoke-Git in parallel by scoop update

Motivation and Context

Closes #5435

How Has This Been Tested?

Used mitmproxy locally to confirm proxy settings are working properly

# disable SSL verification for testing
git config --global http.sslVerify false

# enable proxy settings in scoop
scoop config proxy http://localhost:8080

# run mitmweb
mitmweb

# update scoop and check if everything gets updated properly
./bin/scoop.ps1 update

Checklist:

• I have read the Contributing Guide.
• I have ensured that I am targeting the develop branch.
• I have updated the documentation accordingly.
• I have updated the tests accordingly.
• I have added an entry in the CHANGELOG.

[niheaven]

LGTM, and the chglog is needed.