Fix sporadically failing cache_test.exs
Fix sporadically failing cache_test.exs

This test has been plaguing our CI runs forever by sporadically and
inconsistently failing. The failing test expects a corrupted system
download to be deleted and not exist, but sometimes fails because the
file is there. It turns out that during this particular file check,
we first look for `NERVES_SYSTEM` to be set and use that path if it
exists. We just so happen to set that environment variable in other
tests to a valid system file and there is a race between that env
var being cleaned up and this test running while processing tests
asyncronously, which is why it works for some seeds and not for others.

This forcefully deletes that env var before running the test to ensure
the expected corrupted test file gets picked up by the tooling instead
and cleaned up as expected

nerves-project/nerves,nerves-project/nerves