🧪 [Test]: use `TemporaryEnvironmentVariable` in Arcus.Testing.Core instead

[stijnmoreels]

Is your feature request related to a problem? Please describe.
Currently, we use a custom TemporaryEnvironmentVariable test fixture in the .Tests.Core project, while this is now by default suported in the 📦 Arcus.Testing.Core package.

Describe the solution you'd like
We should use the TemporaryEnvironmentVariable in the 📦 Arcus.Testing.Core package instead, so we don't burden ourselves with supporting additional test fixtures.

Additional context
We should install the v2.x of Arcus.Testing.Core in our .Tests.Core project.

[stijnmoreels]

@joachimgoris , also a good one to pick up.