fix(terraform): add extra handling around private GitHub Terraform modules

[gruebel]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Description

• silenced the Git error log, when using a GitHub PAT, but the normal GitHub module loader tries to download a private module
• added a special override mechanism to be used in the checkov-action to override all GitHub URLs with GitHub PAT, this is really meant as a last resort, because our current code should be enough and is working well ,but I think the misleading error message made user worried it is actually not working
• set GIT_TERMINAL_PROMPT when we do download private modules via GIT
• added a context manager, which gives the possibility to set temporarily an env var

Relates to bridgecrewio/checkov-action#101 and bridgecrewio/checkov-action#129

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my feature, policy, or fix is effective and works
• New and existing tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules