goreleaser snapshot builds

[taylorludwig]

Follows logic of example-auto-snapshot to run a snapshot build when its not triggered from of a tag.

This allows builds to be run during PRs for validatation beforing merging, tagging, and doing an actual release.

Only change to the calling workflow is to add the pull_request trigger eg:

on:
  workflow_dispatch:
  pull_request:
  push:
    tags:
      - v*

You can also manually set the snapshot input to true, false, or auto if the calling workflow wants to sort out its own logic.

Example of a it running on a PR can be seen here:

• Initial PR creation goreleaser release --clean --snapshot
• Updated commit on PR goreleaser release --clean --snapshot
• Final tag creation goreleaser release --clean

[alexlovelltroy]

LGTM