Add fallbacks for primary Button

[langermank]

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What are you trying to accomplish?

Copies the old CSS before the new vars were used. This removes the inset shadow, even though we believe that's the intended design.

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Merge checklist

• Added/updated tests
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• Tested in Chrome
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🦋 Changeset detected

Latest commit: d73f3a3

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/view-components	Patch

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[github-actions]

⚠️ Visual differences found

Our visual comparison tests found UI differences. Please review the differences by viewing the files changed tab to ensure that the changes were intentional.

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