ci: helm-unittest is installed unpinned, so a plugin release can change test results #4510

The unit-test job installs the helm-unittest plugin without a version. .github/workflows/pull-requests.yaml (the "Set up test toolchain" step) runs helm plugin install https://github.com/helm-unittest/helm-unittest with no --version, so every fresh runner takes whatever helm-unittest released last. A new plugin release can then turn chart tests red, or green, on a PR that changed nothing related.

It matters more than it looks, because helm-unittest embeds its own Helm. The current plugin renders with Helm v3.21, while the platform's helm-controller renders with Helm 4.1. A behaviour difference between those two (for example helm/helm#32132, where a null in a parent chart's values is ignored from Helm 4.2 on) is invisible to the tests either way, and a silent plugin bump can move that line without anyone noticing.

Pinning --version (and bumping it on purpose, e.g. through Renovate) would make the test result depend only on the PR.