Passing age keys via stdin SOPS_AGE_KEY_FILE=/dev/stdin not working

Hi,

I want to load the age keys from a password safe (keepassxc) and pass them to sops via stdin

keepassxc-cli show -a password db.kdbx mykey | SOPS_AGE_KEY_FILE=/dev/stdin sops -d secret.yaml 

but I'll get

Failed to get the data key required to decrypt the SOPS file.

Group 0: FAILED
  xxx: FAILED
    - | failed to create reader for decrypting sops data key with
      | age: no identity matched any of the recipients
  
  yyy: FAILED
    - | failed to load age identities: failed to parse
      | 'SOPS_AGE_KEY_FILE' age identities: no secret keys found

Recovery failed because the file was encrypted with a Shamir threshold of
1, but only 0 part(s) were successfully recovered, one for each successful
key group. In order for SOPS to recover the file, at least 1 groups have to
be successful. In order for a group to be successful, decryption has to
succeed with any of the keys in that key group.

Decryption works fine when I write the output from keepassxc-cli show -a password db.kdbx mykey into a file first and use the file path in SOPS_AGE_KEY_FILE

Start by reproducing the keepassxc-cli pipeline with SOPS_AGE_KEY_FILE=/dev/stdin, then compare it with the working file-path form. Trace the SOPS_AGE_KEY_FILE age-identity loading entry point and verify how stdin is read. Done means age keys supplied through stdin are parsed and decryption succeeds for the reported command, with a regression test if the existing test structure supports it.