Allows _GO_SCRIPTS_DIR to be equal to _GO_ROOTDIR

[nkakouros]

• I have reviewed the contributor guidelines.
• I have reviewed the code of conduct.
• Per GitHub's Terms of Service, I am aware that I license my
  contribution under the same terms as this project's license, and
  that I have the right to license my contribution under those terms.

cc: @mbland

What the title says. This adds flexibility to the structure of projects using go-script-bash.

I had encountered a need to set _GO_ROOTDIR to _GO_SCRIPTS_DIR while working on bash-headstart.

[coveralls]

Coverage decreased (-0.05%) to 94.943% when pulling 21e0aaf on tterranigma:rootdir-script-dir into 292e080 on mbland:master.