no error when adding `sops` key to YAML file with `sops edit`

[landure]

I've used sops edit file.yaml to create a YAML file like:

# file.yaml
sops: "something"

After saving the change, sops is unable to decrypt the file: it complains of two sops keys in the YAML.

Please add a check for reserved key names to SOPS, and prevent saving a file with an illegal key name (sops).

[felixfontein]

This also affects sops set: sops set foo.yaml '["sops"]' 123 makes foo.yaml no longer decryptable.

[felixfontein]

I created #2098 to fix both issues.