refactor(test): fix confusing environmental variable names in Rust code

[teor2345]

Motivation

We have Rust variables that store environmental variable names, to detect typos.

But the Rust variables were named differently to the environmental variables, which made them confusing to use.

Solution

fastmod _VAR "" zebra*

Review

Anyone can review this low priority PR.

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

[gustavovalverde]

@teor2345 isn't this causing the issue in https://github.com/ZcashFoundation/zebra/runs/6299601576?check_suite_focus=true#step:8:221 ?

[teor2345]

@teor2345 isn't this causing the issue in https://github.com/ZcashFoundation/zebra/runs/6299601576?check_suite_focus=true#step:8:221 ?

No, the names of the environmental variables have not changed, only the Rust names have.

This PR is not required.