* guix/scripts/publish.scm (narinfo-string): Add #:file-size parameter.
Produce a "FileSize" field when COMPRESSION is eq? to '%no-compression'
or when FILE-SIZE is true.
(bake-narinfo+nar): Pass #:file-size.
* tests/publish.scm ("/*.narinfo")
("/*.narinfo with properly encoded '+' sign")
("with cache"): Check for "FileSize".