SCRIPTS: announce-release: use mutt -H instead of -i to include the draft

SCRIPTS: announce-release: use mutt -H instead of -i to include the draft

Commit 0f5ce6014a ("SCRIPTS: announce-release: place the send command
in the mail's header") broke the announce-release script: by not having
to edit the message at all anymore, mutt does nothing when sending, but
it still does if the message is edited (which was the case before). With
some testing, it appears that mutt -H does work when there's no change,
so let's use this instead. This should be backported till 1.7.