can't pass password via stdin during decrypt

[rdslw]

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 Reading pasted armored input from a terminal clashes with password prompt #364