guix build: Handle "guix build /….drv" correctly for non-existent derivations.

guix build: Handle "guix build /….drv" correctly for non-existent derivations.

This lets the daemon substitute missing derivations, as in the example
at <https://bugs.gnu.org/38226>, instead of failing with ENOENT.

* guix/scripts/build.scm (options->things-to-build): In the
'derivation-path?' case, don't fail when 'read-derivation-from-file'
raises to ENOENT; return the empty list in that case.
(guix-build): Add non-existent '.drv' files to ITEMS.
Pass ITEMS in addition to DRV to 'build-derivations'.
* tests/guix-build.sh: Add test.