flake.lock: update tincr
Picks up the darwin tinc-up fix on lassulus/fixes: route the IPv4 prefix into
the utun. Without it macOS only installed a host route for the tunnel address
and every IPv4 packet for the mesh left through the default gateway. Same bug
kartei carried in its copy of the module (krebs/kartei#65); 28a957a6 switched
barnacle to tincr own module, which never had that fix.