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 https://lab.nexedi.com/nexedi/slapos/merge_requests/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

/cc @xavier_thompson, @jerome