🐛 (svg tester) fix all-views action for the master branch

[sophiamersmann]

• The all-views SVG tester pushes its changes to a branch named <my-branch>__all-views
• If run from the master branch, we want to push to the owid-grapher-svgs repo's master branch, but the action pushes to master__all-views. It also fails with differences on master, because we have some branch-name = 'master' checks (which don't work if the current branch name is master__all-views)
• This is fixed by using the master branch if on master, i.e.:
  • If on master, keep the branch name master
  • If not on master, use <my-branch>__all-views

Note that the SVG tester fails because one of my recent merges into master was supposed to update the owid-grapher-svgs repo but didn't (because the action failed early)

[sophiamersmann]

Current dependencies on/for this PR:

• 🚧 (do not merge) make svg-tester action fail #3252
• 🐛 (svg tester) fix all-views action for the master branch #3251 👈
• master

This stack of pull requests is managed by Graphite.

[danyx23]

Looks good - sorry for not catching that in the original PR!