Run tests modifying DBT_ENV_PRIVATE_GIT_PROVIDER_INFO sequentially

[fornwall]

Fixes #353.

Another alternative to keep the tests as normal tests is to use a mutex (possibly as a macro, as in https://github.com/Thomasdezeeuw/std-logger/blob/17fd34131a1b71063139e393de949a54990b2ef8/src/tests.rs#L14-L33).

[fornwall]

I see now that cargo-nextest is the recommended test runner - and there a process per test is used, which avoids the issues here.

I personally think that is has value to also support normal cargo test if the cost is not huge - so I think it makes sense fixing this.