Fix tpg-transport unit tests for local runs

[ScottSuarez]

A github contributor noted that our unit tests were failing locally for them. Looks like we aren't unsetting the environment variables before performing tests that rely on them to be in a certain state.

Release Note Template for Downstream PRs (will be copied)

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 1 file changed, 37 insertions(+), 10 deletions(-))
Terraform Beta: Diff ( 1 file changed, 37 insertions(+), 10 deletions(-))
TF Conversion: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2852
Passed tests 2551
Skipped tests: 296
Affected tests: 5

Action taken

Found 5 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

TestAccVertexAIIndex_updated|TestAccVertexAIIndex_vertexAiIndexStreamingExample|TestAccVertexAIIndex_vertexAiIndexExample|TestAccContainerAwsNodePool_BetaBasicHandWritten|TestAccComputeFirewallPolicyRule_multipleRules

Get to know how VCR tests work

[modular-magician]

$\textcolor{green}{\textsf{Tests passed during RECORDING mode:}}$
TestAccVertexAIIndex_updated[Debug log]
TestAccVertexAIIndex_vertexAiIndexStreamingExample[Debug log]
TestAccVertexAIIndex_vertexAiIndexExample[Debug log]

Rerun these tests in REPLAYING mode to catch issues

$\textcolor{green}{\textsf{No issues found for passed tests after REPLAYING rerun.}}$

---

$\textcolor{red}{\textsf{Tests failed during RECORDING mode:}}$
TestAccContainerAwsNodePool_BetaBasicHandWritten[Error message] [Debug log]
TestAccComputeFirewallPolicyRule_multipleRules[Error message] [Debug log]

$\textcolor{red}{\textsf{Please fix these to complete your PR.}}$
View the build log or the debug log for each test