Env Var Filter Definition for Debug Artifacts

[mriise]

In the future, we could expand that env var to a set of rules like FVM_STORE_ARTIFACTS=code:SomeCid=/path/to/dir;addr:f01=/path/to/other-dir.

Originally posted by @Stebalien in https://github.com/filecoin-project/ref-fvm/pull/616#discussion_r895215704