refactor(otlp-grpc-exporter-base): use `getStringFromEnv` instead of `process.env`

[weyert]

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Which problem is this PR solving?

Updates the code to user the helper utilities to retrieve environment variable values

Fixes #5560

Short description of the changes

Switched from using process.env to using the helper utility getStringFromEnv

Type of change

Please delete options that are not relevant.

• [X] 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

How Has This Been Tested?

I have manually run the test script command in the package directory

Checklist:

• [X] Followed the style guidelines of this project
• [X] Unit tests have been added
• [ ] Documentation has been updated