Use `withenv` instead of manual setting and re-setting ENV-fix for GR

See https://github.com/trixi-framework/Trixi.jl/blob/5b89962a78bc80de4bbeb8cb1cf2fe7f51aa4338/test/test_examples_2d_advection.jl#L56-L62
It would be cleaner to use `withenv`, maybe even at a higher level

Start in test/test_examples_2d_advection.jl at lines 56-62 and inspect how ENV is manually set and restored for GR. Replace that pattern with withenv, considering whether the environment handling belongs at a higher level. Run the relevant test file and confirm the GR-related examples still pass.

Written by the indexing model from the issue text.