Add helper script to setup envvars via envchain

[annawinkler]

Description

Add a helper script to simplify setting env vars via envchain. Script prompts for an envchain workspace name, then invokes envchain with the environment variables.

Testing plan

1. Pull this branch
2. Run ./scripts/setup-test-envvars.sh
3. If you don't have envchain, the script will exit and request that it be installed via homebrew (this part could be more generic).
4. If you do have envchain, the script prompts for an envchain workspace name, then calls envchain with that workspace name and these 4 envvars: TFE_ADDRESS TFE_TOKEN GITHUB_TOKEN GITHUB_POLICY_SET_IDENTIFIER

Output from tests (HashiCorp employees only)

$ TFE_ADDRESS="https://example" TFE_TOKEN="example" TF_ACC="1" go test ./... -v -tags=integration -run TestFunctionsAffectedByChange

...

[annawinkler]

If we like this script, I can update the testing docs to reference the script.

[uturunku1]

Should we consider alternative solutions to envchain? A concern we have that this script is customized to a very specific tool and other may not end up using this script because everyone ends using their own alternative solution.

[sebasslash]

I won't hit approve cause I'm assuming you're going to address some changes requested by @uturunku1 (so it will dismiss my approval anyways) -- I like the idea of this script, thank you for this 👍

Once they're addressed, everything looks good to me ✅

[github-actions]

Reminder to the contributor that merged this PR: if your changes have added important functionality or fixed a relevant bug, open a follow-up PR to update CHANGELOG.md with a note on your changes.