gnutls: fix musl build
gnutls: fix musl build

`musl` produces a different output than `glibc` during some tests, which
let's them fail.

Using `getpass(3)` under `musl` when `stdin` is not a tty omits the
prompt, which the `certtool` test expects to find.

See https://gitlab.com/gnutls/gnutls/-/issues/945

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs