This will allow mirror operators to alter these non-normative bits of a
narinfo without having to resign narinfos.
* guix/scripts/publish.scm (narinfo-string): Remove
URL/Compression/FileSize from BASE-INFO. Move them after "Signature".
* tests/publish.scm ("/*.narinfo")
("/*.narinfo with properly encoded '+' sign")
("/*.narinfo with lzip + gzip")
("with cache, lzip + gzip"): Adjust accordingly.
* tests/substitute.scm ("query narinfo with signature over relevant subset"):
New test.
