Fix checklist for BCD upgrades workflow

[ddbeck]

A follow up to #2043.

This workflow on the first attempt: https://github.com/web-platform-dx/web-features/actions/runs/11496579249/job/31998472971?pr=2071#logs

Problems:

• No write permissions for the PR
• No checkout to run the diff against (I tested this bit and I have no idea why this worked on my own repo but not here, even though it obviously SHOULD have failed 🙄)

To fix this, I did these things:

• Switched to using https://github.com/dependabot/fetch-metadata instead (to avoid a slow checkout)

  Unfortunately, this makes the PR basically untestable—you can't force Dependabot to do anything. We'll find out if it works tomorrow, I guess.

• Set continue-on-error for both steps, since failures of this workflow shouldn't mark the PR as failing

[ddbeck]

@Elchi3 if you'd have another look at this, I'd appreciate it. 🙏

[Elchi3]

This looks a lot cleaner to me. Fingers crossed this time :)

[Elchi3]

@ddbeck this seems to fail on regular PRs: https://github.com/web-platform-dx/web-features/actions/runs/11499270152

Invalid workflow file: .github/workflows/bcd_upgrade_checklist.yml#L19
The workflow is not valid. .github/workflows/bcd_upgrade_checklist.yml (Line: 19, Col: 13): Unexpected symbol: '"@mdn/browser-compat-data"'. Located at position 62 within expression: contains(steps.dependabot-metadata.outputs.dependency-names, "@mdn/browser-compat-data"