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

**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.

Start in the .Tests.Core project by locating its custom TemporaryEnvironmentVariable fixture and reviewing the Arcus.Testing.Core package reference. Update the project to use Arcus.Testing.Core v2.x instead, remove the redundant fixture usage, and run the .Tests.Core test suite to confirm the tests still pass.

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