snapcraft.yaml: Call mage with "-v" for debugging

See #4538

snapcraft.yaml

@@ -29,7 +29,7 @@ parts:
       export PATH=$GOPATH/bin:$PATH
       cd $GOPATH/src/github.com/gohugoio/hugo
       go get github.com/magefile/mage
-      mage vendor test
+      mage -v vendor test
       rm -f $GOPATH/bin/dep
       rm -f $GOPATH/bin/mage
     install: |