Fix source tag for Snapcraft

Signed-off-by: Chris Warrick <kwpolska@gmail.com>

scripts/release

@@ -90,7 +90,7 @@ sed "s/:Version: .*/:Version: $version/g" docs/*.txt -i
 sed "s/:Version: .*/:Version: Nikola $version/g" docs/man/nikola.rst -i
 sed "s/__version__ = .*/__version__ = '$version'/g" nikola/__init__.py -i
 sed "s/version: .*/version: $version/g" snapcraft/snapcraft.yaml -i
-sed "s/source-tag: .*/source-tag: $version/g" snapcraft/snapcraft.yaml -i
+sed "s/source-tag: .*/source-tag: v$version/g" snapcraft/snapcraft.yaml -i
 sed "s/Nikola==.*/Nikola==$version/g" snapcraft/nikola.py -i
 
 setopt errexit # Exit on errors