chore(ci): ensure stencil nightly gets installed for spec tests (#27974)

Issue number: #

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## What is the current behavior?
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Stencil v3.4 is being used in the nightly run of these tests

## What is the new behavior?
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this PR. -->

this commit updates the `test-core-spec` action to accept an argument
for the version of stencil that should be installed/used. since we do
not (and arguably, should not) cache `node_modules/`, we need a way to
inform this action that a nightly version of stencil should be installed

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information


I fully expect nightly CI to begin to fail as a result of merging this.
We'll fix that next week (in Stencil)
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