Unable to use security key to commit signing using SSH keypair

[Marcos Alano <marcoshalano@gmail.com>]

Hello fellows!

I'm able to sign commits using SSH keypair, but the keypair must be 
located in a file. If I try to use a SSH keypair in a security key (like 
an YubiKey) I get an error. I used this commands to do the test:
```
ssh-keygen -t ed25519-sk -f ~/.ssh/id_ed25519_sk
git config --global gpg.format ssh
git config --global user.signingkey "$(cat ~/.ssh/id_ed25519_sk.pub)"
git commit -S --allow-empty --message="Testing"
```

Bnd I get this error:
```
error: Couldn't load public key sk-ssh-ed25519@openssh.com <my key id>: 
No such file or directory?

fatal: failed to write commit object
```
I did the same thing with a plain ed25519 keypair and worked.

Am I doing anything wrong or security keys aren't supported yet?

Thank you for any help,
-- 
Marcos Alano