chore(ci): core build caches api.txt (#27167)

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->


<!-- Issues are required for both bug fixes and features. -->
Unbuilt changes in `core` are not always being caught because the
`api.txt` file is not cached.

Stencil will potentially update both the `src/components.d.ts` and
`api.txt` files when there are built changes to the public API. We need
to check both files when accounting for unbuilt changes.

Example: https://github.com/ionic-team/ionic-framework/pull/25858

This had updates to `api.txt` that were not built.

Built changes PR:
https://github.com/ionic-team/ionic-framework/pull/25933

Example of this change catching `api.txt` changes as intended:
https://github.com/ionic-team/ionic-framework/pull/27165

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- GitHub Actions core build step caches `api.txt`

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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screenshots of how the component looks before and after the change. -->

.github/workflows/actions/build-core/action.yml

@@ -22,4 +22,4 @@ runs:
       with:
         name: ionic-core
         output: core/CoreBuild.zip
-        paths: core/dist core/components core/css core/hydrate core/loader core/src/components.d.ts
+        paths: core/dist core/components core/css core/hydrate core/loader core/src/components.d.ts core/api.txt