For a flag type used for attributes/secrets, values have always been
able to be used with env:// or file:// format, but the error check to
see whether the value was successfully parsed was doing a simple string
equality rather than `errors.Is`. As a result a value containing a colon
would trip up this check and cause an error, rather than allowing it to
be used as a bare value.
Jeff Mitchell
f1b737c62e