This filters the secrets to only include the secrets managed under `per-machine` and `shared`,
otherwise new deployments will fail, when using the vars module for multiple machines:
```
[vyr] /nix/store/[…]sops-install-secrets: failed to decrypt '/nix/store/[…]/sops/vars/per-machine/draper/garage/admin_token/secret': Error getting data key: 0 successful groups required, got 0
```
This doesn't fix all the edge cases with this approach.
We get a similar error if we deploy shared vars that are not
encrypted for our machine key. This needs to be addressed when
implementing the shared vars functionality.
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