build: ignore github actions from dependabot

[ktmud]

SUMMARY

Ask Dependabot to ignore .github/actions since the dependencies in the local copy of external actions should be manually updated when we update the action.

Related: #12241

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TEST PLAN

CI must pass.

[robdiciuccio]

Does this prevent Dependabot security notifications for Github Actions, or just the automated update PRs? It's not very clear from the docs.

[ktmud]

@robdiciuccio I think this does affect security notifications:

Any options that also affect security updates are used the next time a security alert triggers a pull request for a security update.

But I just found out you can choose to keep the checks but disable pull requests: https://docs.github.com/en/free-pro-team@latest/github/administering-a-repository/configuration-options-for-dependency-updates#open-pull-requests-limit

I've updated the PR accordingly. Thanks for pointing this out!

[mistercrunch]

LGTM