Not sure what the problem is exactly. If I follow the instructions (except that I do not run setup_go_vars entirely, just set GOPATH, we should probably improve that part), it builds correctly:

matthijs@grubby:/tmp$ git clone https://github.com/arduino/arduino-builder.git
Cloning into 'arduino-builder'...
remote: Counting objects: 4953, done.
remote: Total 4953 (delta 0), reused 0 (delta 0), pack-reused 4953
Receiving objects: 100% (4953/4953), 1.14 MiB | 977.00 KiB/s, done.
Resolving deltas: 100% (3131/3131), done.
matthijs@grubby:/tmp$ cd ar=
-bash: cd: ar=: No such file or directory
matthijs@grubby:/tmp$ cd arduino-builder/
matthijs@grubby:/tmp/arduino-builder$ export GOPATH=$(pwd)
matthijs@grubby:/tmp/arduino-builder$ go get github.com/go-errors/errors
matthijs@grubby:/tmp/arduino-builder$ go get github.com/stretchr/testify
matthijs@grubby:/tmp/arduino-builder$ go get github.com/jstemmer/go-junit-report
matthijs@grubby:/tmp/arduino-builder$ go build arduino.cc/arduino-builder
matthijs@grubby:/tmp/arduino-builder$ ls
arduino-builder  bin  CONTRIBUTING.md  fmt_fix_vet  LICENSE.txt  pkg  README.md  setup_go_env_vars  src

You'll have to provide more details about how you set things up. Also, I would suggest you reconsider your (even slightly) snarky tone, which seems inappropriate to me.

You are right. Please forgive.
The procedure was failing without the export GOPATH=$(pwd) line that you just shown.
(ubuntu64 4.4.0-92-generic )

go version
go version go1.9 linux/amd64u

zen@book:$ git clone https://github.com/arduino/arduino-builder.git
Cloning into 'arduino-builder'...
remote: Counting objects: 4953, done.
remote: Total 4953 (delta 0), reused 0 (delta 0), pack-reused 4953
Receiving objects: 100% (4953/4953), 1.14 MiB | 797.00 KiB/s, done.
Resolving deltas: 100% (3131/3131), done.
Checking connectivity... done.
zen@book:$ cd arduino-builder
zen@book:/arduino-builder$ ls
CONTRIBUTING.md fmt_fix_vet LICENSE.txt README.md setup_go_env_vars src
zen@book:/arduino-builder$ export GOPATH=$(pwd)
zen@book:/arduino-builder$ go get github.com/go-errors/errors
zen@book:/arduino-builder$ go get github.com/stretchr/testify
zen@book:/arduino-builder$ go get github.com/jstemmer/go-junit-report
zen@book:/arduino-builder$ go build arduino.cc/arduino-builder
zen@book:~/arduino-builder$ ls
arduino-builder CONTRIBUTING.md LICENSE.txt README.md src
bin fmt_fix_vet pkg setup_go_env_vars

So, thanks.
The tool is also included in the arduino 1.8.4 package.
This tool is finally just a wrapper script for avrdude? Isn'it?

No, avrdude (which handles uploading) is directly called by the Arduino IDE. arduino-builder is in charge of calling avr-gcc and some other tools to compile a sketch. Since this is fixed for you, I'll mark this issue as closed. I've just created #245 with some improvements to the README which might have prevented your issue, feel free to see what you think of them.