guix build: '-f' accepts file-like objects.

* guix/scripts/build.scm (options->things-to-build)[validate-type]:
Check for 'file-like?'.
(options->derivations): Accept 'file-like?'.
* tests/guix-build.sh: Add a test with 'computed-file'.
* doc/guix.texi (Additional Build Options): Mention file-like objects.