Ensure error messages don't leak private key

Since #8766, invalid base64 is rendered in errors, but we don't actually
want to show this in the case of an invalid private keys.

Co-Authored-By: Eelco Dolstra <edolstra@gmail.com>
(cherry picked from commit 2b6b03d8df)

src/libstore/ssh.hh

@@ -14,6 +14,9 @@ private:
     const std::string host;
     bool fakeSSH;
     const std::string keyFile;
+    /**
+     * Raw bytes, not Base64 encoding.
+     */
     const std::string sshPublicHostKey;
     const bool useMaster;
     const bool compress;