For any Spack test using Spack's YAML configuration, avoid using real
Spack configuration that has been cached by other tests and Spack
startup logic. Previously this was only done for tests using
'mutable_config' (i.e. those which expected to *change* the
configuration of Spack), but in fact all tests that read Spack config
should use it.
This was an issue when running tests within an environment, because
compiler configuration ends up being queried earlier, and the user's
real config "leaks" into the cache. Outside an environment, the cache
is never set until tests touch it, so we weren't seeing this issue.
54a6c25da6