fix: inject snap package version from git tag

[rianjs]

Summary

• Snap packages show versions like 0+git.9337614 instead of proper semver in the Snapcraft Store
• Root cause: version: git in snapcraft.yaml relies on git describe --tags which fails inside the LXD build container
• Fix: inject version from git tag into snapcraft.yaml before snapcore/action-build runs

Fixes #80

Test plan

• Verify CI passes
• Next release will publish snap with proper semver version