golang: Fix build to be robust to $GOBIN set from outside

Xavier Thompson reports that when Theia is compiled inside Theia, Go
fails to install with the error showing that golang14's make.bash calls
`go install` to install cmd/go and other commands to the outer
go.work/bin instead of inside $GOROOT_FINAL set by the build recipe.

Similarly to nexedi/slapos!928 this
might be to the fact that internal `go install` prioritizes $GOBIN even
in the presence of set $GOROOT_FINAL.

-> Don't try to fix Go itself, and make the compilation more robust by
unsetting $GOBIN, so that even if that variable is set in the
environment where buildout runs, it does not affect the compilation
procedure.

Fixes: 8eac67a5 (golang: Prepare for future GOPATH removal)
Reported-on: https://www.erp5.com/group_section/forum/Go-compilation-error-P6f5WisQkc
Tested-by: @xavier_thompson
Reviewed-on: nexedi/slapos!932