snap: Fix snapcraft builds #12879

* Use upstream git repo as snap source.
* Fix typo in snapcraft.yaml
  Use latest_tag instead of branch because branch is undefined.

snap/snapcraft.yaml

@@ -25,7 +25,7 @@ apps:
 
 parts:
     nvim:
-        source: .
+        source: https://github.com/neovim/neovim.git
         override-pull: |
           snapcraftctl pull
           latest_tag="$(git tag -l --sort=refname|head -1)"
@@ -37,9 +37,9 @@ parts:
           git_described="$(git describe --first-parent --dirty 2> /dev/null | perl -lpe 's/v\d.\d.\d-//g')"
           git_described="${git_described:-$(git describe --first-parent --tags --always --dirty)}"
           if [ "${version_prefix}" != "${git_described}" ]; then
-            VERSION="${version_prefix}-${git_described}-${branch}"
+            VERSION="${version_prefix}-${git_described}-${latest_tag}"
           else
-            VERSION="${version_prefix}-${branch}"
+            VERSION="${version_prefix}-${latest_tag}"
           fi
           snapcraftctl set-version "${VERSION}"
         plugin: make