[Snap] Set the snap package version using git tag instead of VERSION file

[chanchiwai-ray]

The snapcraft.yaml uses the VERSION file to indicate the version of the snap package, but this file cannot reflect the latest version of the snap package correctly if there are patches added the in the latest release. For example, if the VERSION is set to 1.7.0, and there are some patches added after the release, the snap should be versioned with 1.7.0.xxxx.

This can be achieve using the override-pull in snapcraft.yaml: (note it requires the repo to fetch the git tags)

...
    override-pull: |
      craftctl default
      TAG=$(git describe --tags --abbrev=10)
      craftctl set version="${TAG#v}"
...

Related to #346