tools: Allow versions to either have a v prefix or not

Sometimes it's v0.9.3 and sometimes it's 0.9.3, and I always get it
wrong. This is painful when you have to create a signed tag after
committing to one of the two only to find out you chose wrong and have
to redo the tagging.

tools/build-release.sh

@@ -66,7 +66,7 @@ if [ "$VERSION" = "" ]; then
 fi
 
 # Skip 'v' here in $VERSION
-MTIME=${FORCE_MTIME:-$(sed -n "s/^## \\[${VERSION#v}\\] - \\([-0-9]*\\).*/\\1/p" < CHANGELOG.md)}
+MTIME=${FORCE_MTIME:-$(sed -n "s/^## \\[.*${VERSION#v}\\] - \\([-0-9]*\\).*/\\1/p" < CHANGELOG.md)}
 if [ -z "$MTIME" ]; then
     echo "No date found for $VERSION in CHANGELOG.md" >&2
     exit 1