guix: scripts: Add GUIX_BUILD_OPTIONS environment handling.
* doc/guix.texi (Invoking guix build): Mention 'GUIX_BUILD_OPTIONS'.
* guix/scripts/archive.scm: (append args (environment-build-options)).
* guix/scripts/build.scm: Ditto.
* guix/scripts/environment.scm: Ditto.
* guix/scripts/package.scm: Ditto.
* guix/scripts/system.scm: Ditto.
* guix/ui.scm (environment-build-options): New function.
* guix/utils.scm (arguments-from-environment-variable): New function.
* tests/guix-build.sh: Add tests.
* test-env.in: Unset GUIX_BUILD_OPTIONS.
Co-authored-by: Ludovic Courtès <ludo@gnu.org>
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