workflows/docker: continue to deploy master images

[ZhongRuoyu]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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I noticed that master images have not been updated since #18912:

$ docker run --pull=always --rm -it homebrew/brew:master brew --version
master: Pulling from homebrew/brew
Digest: sha256:3812ffd9b728ce3d96a2a362ef33bed420d1dc73c7d96c93a8f8d2d4f10e6281
Status: Image is up to date for homebrew/brew:master
Homebrew 4.4.11-6-geae8d1b
Homebrew/homebrew-core (git revision 9610909d254; last commit 2024-12-10)

That is due to images no longer being built on master pushes. This change restores the previous behaviour.