🧪 [Test]: use Arcus.Testing's `TemporaryEnvironmentVariable` test fixture instead of implementing ourselves

[stijnmoreels]

Is your feature request related to a problem? Please describe.
Currently, the code implements a test fixture to temporary set an environment variable during testing. This functionality is already built-in available in Arcus.Testing.

Describe the solution you'd like
Use the TemporaryEnvironmentVariable test fixture in the 📦 Arcus.Testing.Core package instead of implementing it ourselves.

Additional context
This test fixture is located in the Arcus.Observability.Tests.Core project.