Don't delete encryption password until volume deletion succeeds

[cole-h]

Description

If the encryption key is deleted, the volume is useless until the user figures out they have to delete it. It is not safe for us to decide to delete it.

Instead, we should not delete the encryption key until we are confident the volume itself was deleted.

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

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curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install

[grahamc]

Looks good. I asked in a DM about if you'd considered a match on self.create_volume.try_revert().await instead of reassigning the bool, but its really a nit.