ci: split examples tests (#9729)
### Description
Currently, our examples testing loop is too slow. Changing just one of
the examples causes all of them to miss cache.
This PR instead runs tests only for changed examples. This is going to
end up being an even larger improvement given that I've additionally set
up a test for each package manager for each core-maintained example,
effectively tripling the amount of testing needed.
### Notes
This probably isn't the most elegant way to write this - but it was the
easiest to understand. GitHub Actions is just...really hard to work with
and my patience has run out. Maybe I'll clean this up later, or maybe
some kind soul will review me and give me a quick way to DRY it up.
- The additions in `.github/workflows` are all duplicates of each other,
but one for each example.
- The additions in `turborepo-tests/example-*` are creating tests for
all of our supported package managers where we used to only test one
package manager per example.
### Testing Instructions
CI
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Co-authored-by: Chris Olszewski <chris.olszewski@vercel.com>
