Create Plugin: Fix CI workflow for E2E tests to successfully run

[jackw]

What this PR does / why we need it:

This PR fixes the following issues with a newly scaffolded plugin failing to run e2e tests:

1. The CI workflow currently has a hardcoded backend file name when trying to chmod the files. This PR addresses this by using a wildcard to chmod all files in ./dist that start gpx_.
2. The CI workflow currently only downloads the built plugin artefacts if there is a backend component. This PR addresses this by removing the if logic in the step.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

📦 Published PR as canary version: Canary Versions

✨ Test out this PR locally via:

npm install @grafana/create-plugin@4.16.2-canary.1001.ebc5fe7.0
# or 
yarn add @grafana/create-plugin@4.16.2-canary.1001.ebc5fe7.0

[github-actions]

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[grafana-plugins-platform-bot]

🚀 PR was released in @grafana/create-plugin@4.16.2 🚀