Allow Secrets Manager to set via environment variable name

[gaffneyc]

Setting up fnox for the first using AWS Secrets Manager and I keep getting tripped up by how it references the key names for secrets manager.

I expect I can use the env variable names (e.g. fnox set NOTARY_KEY) but that returns an error since it maps to signing/NOTARY_KEY instead of the value defined in fnox.toml. I have do the mental mapping from NOTARY_KEY, remove the prefix, then run fnox set notary-key instead.

It seems like fnox should be able to map the environment variable to the key being set so I don't have to.

$ base64 -w0 notary.p8 | fnox set NOTARY_KEY
Error: fnox::provider::secret_not_found (https://fnox.jdx.dev/providers/aws-sm)

  × AWS Secrets Manager: secret 'NOTARY_KEY' not found
  help: The secret 'NOTARY_KEY' does not exist in AWS Secrets Manager.
        Check that the secret exists in AWS Secrets Manager

fnox.toml

if_missing = "error"

[providers.sm]
type   = "aws-sm"
region = "us-east-1"
prefix = "signing/"

[secrets]
NOTARY_KEY = { provider = "sm", value = "notary-key" }