Add support for login via creds file to non-prod envs

[ncabatoff]

Changes proposed in this PR:

If a credentials file is used for login, the env vars were ignored, making it impossible to test in e.g. a PRDE. Add a call to FromEnv when using creds file.

How I've tested this PR:

I was trying to test logging in using WIF, and it wasn't finding my WIF provider resource. Debugging revealed it was connecting to the prod hcp, even when I set the appropriate env vars. After I made this change and rebuilt, the resource in my PRDE was found and I was able to complete the login to my PRDE using a creds file.

How I expect reviewers to test this PR:

Checklist:

• Tests added if applicable
• CHANGELOG entry added or label 'pr/no-changelog' added to PR

  Run CHANGELOG_PR=<PR number> make changelog/new-entry for guidance
  in authoring a changelog entry, and commit the resulting file, which should
  have a name matching your PR number. Entries should use imperative present
  tense (e.g. Add support for...)