GitHub Actions: Replace master references with origin/HEAD

[sarayourfriend]

In many cases, origin just by itself is an alias for origin/HEAD.
When a specific branch name is needed, such as in the case of the
CI caches changed in this commit, we can use HEAD as a direct
alias for the primary branch, regardless of to what it is currently
set.

See set-head entry in git remote man page: https://mirrors.edge.kernel.org/pub/software/scm/git/docs/git-remote.html

Our repository is configured with HEAD -> master:

$ git branch -l -a | grep remotes/origin/HEAD
remotes/origin/HEAD -> origin/master

Changes proposed in this Pull Request

• Update GitHub Action references to master branch with references to origin/HEAD or just origin or HEAD when appropriate

Testing instructions

• Check that GitHub actions on this PR continue to work

Part of #43939

[matticbot]

Test live: https://calypso.live/?branch=update/reference-origin-head-rather-than-branch-GitHubActions

[noahtallen]

Wow, nice! Thank you for doing this. I didn't know this was a thing :)

To cause the FSE plugin tests to run, could you commit a small change to a file in apps/full-site-editing/full-site-editing-plugin/?

[sarayourfriend]

There's also this line in the ICFY action but it cannot be changed to use origin/HEAD: https://github.com/Automattic/wp-calypso/blob/95211b413b3b9c8cb61c2f94ecb80d62bffc6bfe/.github/workflows/icfy-stats.yml#L45

I suspect this is due to how the checkout action works which is that it inits an empty repository and then add the origin remote rather than just cloning the repository. I think this causes it to lose the mapping between HEAD -> master that exists normally.

From tmate action debugging session:

runner@fv-az32:~/work/wp-calypso/wp-calypso$ git merge-base HEAD remotes/origin/HEAD
fatal: Not a valid object name remotes/origin/HEAD
runner@fv-az32:~/work/wp-calypso/wp-calypso$ git branch -l -a | grep HEAD
runner@fv-az32:~/work/wp-calypso/wp-calypso$ git branch -l -a | grep master
  remotes/origin/master

This will just need to be changed when we actually rename the branch to trunk.

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[matticbot]

Caution: This PR affects files in the FSE Plugin on WordPress.com
Please ensure your changes work on WordPress.com before merging.

D46056-code has been created so you can easily test it on your sandbox. See this FieldGuide page about developing in the FSE Plugin for more info: PCYsg-ly5-p2

[noahtallen]

The changes to the FSE plugin workflow look good to me!