envknob: remove some stutter from error messages

The strconv errors already stringified with the same.

Change-Id: I6938c5653e9aafa6d9028d45fc26e39eb9ccbaea
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f