provide auto-fix patch file (add `fix-patch-path` output)

[2bndy5]

The idea for this is related to

• how to commit changes if checks fail (without using PR review suggestions) cpp-linter#82

Here's a flowchart of the changes in #358

flowchart TD;
  isTidy{Is clang-tidy desired?}
  runTidy[Run clang-tidy]
  cacheTidy[Save clang-tidy fixes to cache]
  isFmt{Is clang-format desired?}
  runFmtOg[Run clang-format on event src file]
  hasTidyFixCache{Has clang-tidy cached any fixes?}
  do3wayDiff[Normalize event changed lines with clang-tidy fixed lines]
  runFmtTidy[Run clang-format on clang-tidy fixes]
  cacheFmt[Save clang-format fixes to cache]

  isTidy --yes--> runTidy --> cacheTidy --> isFmt;
  isTidy --no --> isFmt --yes--> runFmtOg --> hasTidyFixCache --no --> cacheFmt;
  hasTidyFixCache --yes--> do3wayDiff --> runFmtTidy;

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What's next?

Now it is just a matter of presenting a patch file that users can consume.

My instinct says: Save the patch file in .cpp-linter-cache/auto-fix.patch. Then set a new output variable fix-patch-path pointing to the saved patch file.

From there users can save it as an artifact or do whatever with it. Using the patch file as an artifact in a different job allows for isolated contents: write permissions (needed by git push after doing git apply $PATCH_FILE).