clan-cli: filter any sops recipients set in the environment for encryption

This forces sops to use our config file, otherwise if any of the
environment variables set to specify recipients is present then
`--config` will be ignored (see [env_check]).

That's simple enough, still I ended up refactoring how we call sops for
correctness, and to align with its behavior. The code now distinguishes
between public and private keys explicitly. `secrets.decrypt_secret`
does not try to lookup for public and private keys anymore.

With this changeset, some people might have to adjust their environment
as public age and PGP keys will be discovered like sops would do. In
particular if multiple public keys are discovered, then the user will
have to specify which one to use for the clan.

This also makes the following changes:

- try to use `/dev/shm` when swapping a secret (it's what [pass] does
  fwiw);
- alias immediate values for readability;
- remove some float comparison that could never succeed, and use sops'
  exit status instead;
- remove unused function `maybe_get_sops_key`.

[env_check]: 8c567aa8a7/cmd/sops/main.go (L2229)
[pass]: http://passwordstore.org/

pkgs/clan-cli/clan_cli/machines/update.py

@@ -114,8 +114,8 @@ def deploy_machine(machines: MachineGroup) -> None:
 
     def deploy(machine: Machine) -> None:
         host = machine.build_host
-        generate_facts([machine])
-        generate_vars([machine])
+        generate_facts([machine], service=None, regenerate=False)
+        generate_vars([machine], generator_name=None, regenerate=False)
 
         upload_secrets(machine)
         upload_secret_vars(machine)