guix: scripts: Parse $GUIX_BUILD_OPTIONS separately.

Appending to "raw" args broke optional parameters in 'guix package -I'
and 'guix package -A', and possibly other places.  Therefore, switch to
parsing each set of options on its own and append resulting alists
together afterwards.

* guix/scripts/archive.scm (parse-options-from): Rename from
  (parse-options) and add explicit argument.  New form of (parse-options)
  using its old algorithm via -from function.
* guix/scripts/build.scm: Ditto.
* guix/scripts/environment.scm: Ditto.
* guix/scripts/package.scm: Ditto.
* guix/scripts/system.scm: Ditto.
* tests/guix-package.sh: Add test.
* doc/guix.texi (Invoking guix build): Make it clear that the options
  are parsed independently.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>