Fix a stray reference to ${os} in build/toolchain/nacl/BUILD.gn.
Fix a stray reference to ${os} in build/toolchain/nacl/BUILD.gn.

The 'os' variable is going away and needs to be replaced with
'current_os'.

R=ncbray@chromium.org
BUG=344767

Review URL: https://codereview.chromium.org/1019643005

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client