fix(webapp): disable browser autofill on environment variable inputs (#4777)

The environment variable key and value inputs did not set an
autocomplete attribute, so browsers could offer to autofill or save
typed values as saved credentials. This sets `autoComplete="off"` on
those inputs in both the create and edit forms, matching the
`autoComplete="off"` convention already used on the other
credential-name inputs.

`autoComplete="off"` is a best-effort hint. Browsers may still ignore it
for password-typed fields, so this is defense-in-depth hardening, not a
hard guarantee that a password manager cannot store the value.