ci/tests: test environment should ignore current user's global/system configs

[pmrowla]

When we run our unit/func test suite it should run in a completely clean & reproducible environment - meaning the test environment should also ignore the running user's global and system level DVC configs if any exist.

• For disabling the user's global config we should just override $HOME (or the windows equivalent env vars) so that it points to an empty temp directory (or some default template dir that we set up explicitly for our CI/test environment).
• For system configs we could introduce something similar to git's $GIT_CONFIG_NOSYSTEM env var, and enable it in our test environment

[karajan1001]

Agreed. I used to work on a dvc config PR last year, and suffer a lot from global config caused problem when running tests.