Always allow base64 encoded flag for environment variable based values

As an example, this is what Acquia says when trying to store the pem key value.

Here's a patch that removes the check.

I needed the same functionality and agree I see no downside in allowing this option for all environment variables.

I re-rolled the patch as an MR and adjusted the description language to clarify it. I feel like "NB" is not clear to everyone, and as per the discussion in #2760911: Use of "e.g." and "i.e." → I've changed it from a Latin abbreviation to "Note:"