can't pass password via stdin during decrypt#603

## Environment * OS: Linux 6.9.11 * age version: 1.2.0 ## PROBLEM Currently (afaik) it's not possible to pass passphrase to age via stdin, like: `echo -n "$passphrase" | age -d -o output_file input_file` Here, age knows that stdin is NOT used for data, because I specified input file explicitly. This also does not work with: `some_cli_password_manager | age -d -i - -o output_file input_file` Rationale: 1. passphrase is eventually read from stdin 2. sometimes you must use passswords (can't use keys) in script & automation 3. most cli tools allows it (cryptsetup etc.) 4. also #364