bug: `--check-cache-status` false positive

[nrdxp]

I'm not really sure the exact semantics of the bug just yet, but when I dump a flake's nixConfig into the environment via the NIX_CONFIG variable, I get false positives when passing --check-cache-status, that is, isCached is reporting true even though the path is not cached in any of the substituters.

Not sure, but does the code consider the local /nix/store a substituter? That might be a possible explanation since the derivation in question is a NixOS config that I had to rebuild to modify my /etc/nix.conf to try and triage this issue, but it probably shouldn't since I think we want this flag to report if an artifact exists on a remote.

[colemickens]

from the help text:

  --check-cache-status   Check if the derivations are present locally or in any configured substituters (i.e. binary cache). The information will be exposed in the `isCached` field of the JSON output.

I've taken to just trying to push any paths that exist locally, as it means that I might've (just) built them locally.