🐛 (svg tester) fix actions when running in parallel

[sophiamersmann]

Problem

• Both actions commit and force-push changes to master
• As a result, the last push wins and overwrites the commit of the sibling action

Solution

• Rather than directly committing to master, the actions create a temporary branch called master__<unix timestamp> and force-push to that branch
• If there are differences, we want to merge the temporary branch into master
• To do so, we switch to master, pull its current version from remote, merge the temporary branch, and force-push the result
  • If we do get a merge conflict, it's automatically resolved by keeping their version, i.e. the version of the temporary branch the contains the differences

[sophiamersmann]

• 🐛 (svg tester) fix actions when running in parallel #3276 👈
• master

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[danyx23]

Looks good! 🤞