build: Move metalint to a separate build step (and add build step timings)
build: Move metalint to a separate build step (and add build step timings)

I run a lot of builds. They're quite slow now:

    jb@syno:~/s/g/s/syncthing $ BUILDDEBUG=1 ./build.sh
        ... snipped commands ...
    runError: gometalinter --disable-all --deadline=60s --enable=varcheck . ./cmd/... ./lib/...
    ... in 13.00592726s
    ... build completed in 15.392265235s

That's 15 s total build time, 13 s of which is the varcheck call. The
build server is welcome to run it, but I don't want to on each build. :)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3285

rumpelsepp/syncthing,TheBigBear/syncthing,plouj/syncthing,syncthing/syncthing,wweich/syncthing,kluppy/syncthing,rumpelsepp/syncthing,uok/syncthing,letiemble/syncthing,jryans/syncthing,rumpelsepp/syncthing,imsodin/syncthing,nrm21/syncthing,alex2108/syncthing,Kudalufi/syncthing,cherylquirion/syncthing,AudriusButkevicius/syncthing,kamadak/syncthing,wweich/syncthing,plouj/syncthing,letiemble/syncthing,xduugu/syncthing,Kudalufi/syncthing,wweich/syncthing,lkwg82/syncthing,brgmnn/syncthing,le9i0nx/syncthing,lkwg82/syncthing,Kudalufi/syncthing,ProactiveServices/syncthing,uok/syncthing,lkwg82/syncthing,imsodin/syncthing,kamadak/syncthing,jryans/syncthing,nrm21/syncthing,TheBigBear/syncthing,imsodin/syncthing,syncthing/syncthing,le9i0nx/syncthing,kluppy/syncthing,letiemble/syncthing,xduugu/syncthing,ProactiveServices/syncthing,xduugu/syncthing,syncthing/syncthing,AudriusButkevicius/syncthing,TheBigBear/syncthing,lkwg82/syncthing,le9i0nx/syncthing,alex2108/syncthing,alex2108/syncthing,brgmnn/syncthing,AudriusButkevicius/syncthing,imsodin/syncthing,rumpelsepp/syncthing,cherylquirion/syncthing,kluppy/syncthing,imsodin/syncthing,nrm21/syncthing,cherylquirion/syncthing,plouj/syncthing,uok/syncthing,plouj/syncthing,jryans/syncthing,letiemble/syncthing,alex2108/syncthing,kamadak/syncthing,syncthing/syncthing,ProactiveServices/syncthing,AudriusButkevicius/syncthing,syncthing/syncthing,le9i0nx/syncthing,brgmnn/syncthing,xduugu/syncthing,ProactiveServices/syncthing,TheBigBear/syncthing,cherylquirion/syncthing,kamadak/syncthing,wweich/syncthing,uok/syncthing,kluppy/syncthing,jryans/syncthing,nrm21/syncthing,Kudalufi/syncthing,brgmnn/syncthing,syncthing/syncthing