Stop updating v1 tag, encourage referencing specific version instead

[robin-aws]

The latest change (#8) broke the CI for https://github.com/dafny-lang/libraries because it was already installing the report generator itself, and dotnet tool install is not idempotent.

Ignoring for the moment whether this should have been considered a breaking change, it's really unfortunate for consumers to potentially break spontaneously when we publish a new version. I would strongly prefer to encourage users to reference a specific version (i.e. @v1.4.0 instead of @v1) and rely on dependabot to keep the version up to date instead: https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

We will need to update the sample in the README accordingly, and warn about this change. Ideally we would remove the v1 tag entirely.