build: distribute signed sha256sums

build: distribute signed sha256sums

Distribute clearsigned sha256sum file in addition to the detached
signature.

Verifies that use the sha256sum ensure that the thing signed includes
the name of the tarball. This defends the verifier by default against
a freeze, rollback, or project substitution attack.

A verifier can use something like the following (as expressed in
bash):

      set -o pipefail
      wget https://notmuchmail.org/releases/notmuch-$VERSION.tar.gz{,.sha256.asc}
      gpgv --keyring ./notmuch-signers.pgp --output - notmuch-$VERSION.tar.gz.sha256.asc | sha256sum -c -

See id:87r2b8w956.fsf@fifthhorseman.net and other messages in that
thread for discussion.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>