agent: Fix regression in recent ssh changes.

* agent/command-ssh.c (sexp_key_construct): Lowercase the algo name.

We need to use a lowercase version of the algo in S-expression.
Unfortunately Libgcrypt has no function for this, thus we need to
malloc and first.

• Fixes-commit: ebf24e3
• Signed-off-by: Werner Koch <wk@gnupg.org>