[CI] Change preview.yml to validate.yml

[dmcardle]

This change converts the "Preview" job into a "Validate" job. It still runs for every PR, but it does not deploy to the gh-pages branch.

Fixes #148

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Preview | Diff

[dmcardle]

Added a temporary change to spec.bs to exercise the "Validate" job and saw what I expected:

Deploy to GitHub pages
  Skipped.

I'll drop that temp commit now.

[dmcardle]

@alexmturner Low priority — PTAL when you get a chance.

[alexmturner]

LGTM!