Restore environment variables to original state after tests

[emily-emily]

Problem

The current tests manipulate environment variables and do not restore the original values after execution. This can cause problems for subsequent tests, eg. when the env var overrides something important or is intentionally set to an invalid value for the test.

Solution

This PR introduces the temp_env crate, which restores original environment variable values after the given scope. It uses a mutex to lock the variables so they cannot be concurrently accessed during the tests.

Test cases were also updated to to use a Result return type.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

All tests pass with the updated environment variable code.

[austin-denoble]

Nice work, and nice considerations around the approach to testing and considering test pollution. I think given what we're trying to test in terms of creating a new Pinecone struct this makes a lot of sense.

For reference, I had to do something similar with environment variables in our Go tests to make sure I was setting something for the test, and then unsetting before continuing: https://github.com/pinecone-io/go-pinecone/blob/257e9f6453ff41ce32bca1eff55687ef96432bab/pinecone/client_test.go#L237

[ericapywang]

looks good