refactor(ci): drop bcgov/action-get-pr from the .deployer.yml reusable workflow #53

Problem

The .Helm Deployer reusable workflow still uses the pre-consolidation bcgov/action-get-pr@28b0adf… # v0.3.1 to work out the default tag (the PR number) when tag is empty. bcgov/actions deprecated get-pr in favour of image-tracker (bcgov/actions#200), but image-tracker isn't a drop-in replacement here. It only returns a PR number while resolving package images.

Affected

• .github/workflows/.deployer.yml (~line 98)

Found while removing old-repo dependencies in #48, which leaves this one in place.

Done when

• .deployer.yml no longer depends on bcgov/action-get-pr. For example, use github.event.number on pull_request events and require tag otherwise, or resolve images with bcgov/actions/image-tracker and use its pr output.
• The behaviour for merge/push callers is documented.