Copied part of gowork.install:command to activate GO111MODULES and also the
[caddy] section is minimised, as the whole build happnes in proper place.

Should be used soon.

There was missing promise from other work.

See merge request nexedi/slapos!814

This is critical feature of SlapOS Master instantiation, and very easy to
damage, thus testing it is important.

It's useful for some tests to use.

Some profiles needs to extends the path with binaries location of provided
software.