Explicitly set permissions on user and host keys.

On cygwin, the umask might not be sufficient.  Should fix tests on
Github runners.

regress/test-exec.sh

@@ -674,6 +674,9 @@ for t in ${SSH_HOSTKEY_TYPES}; do
 
 	# don't use SUDO for proxy connect
 	echo HostKey $OBJ/$t >> $OBJ/sshd_proxy
+
+	# explicitly set perms since cygwin inhertited perms might be too open
+	chmod 600 $OBJ/$t $OBJ/host.$t
 done
 chmod 644 $OBJ/authorized_keys_$USER