fix model-transparency GitHub pages via workflow build type

[spencerschrock]

Summary

The legacy build type requires the gh-pages branch to exist for GitHub Pages to be enabled. We're deploying with a workflow anyway, so this better matches our setup.

See also: 96a204a, #480

Release Note

NONE

Documentation

NONE

[github-actions]

🍹 preview on sigstore-github-sync/sigstore/github-prod

Pulumi report

  Previewing update (sigstore/github-prod)

View Live: https://app.pulumi.com/sigstore/sigstore-github-sync/github-prod/previews/c4d83e54-bce9-4ac9-bce5-82623466f52d

@ Previewing update.....
pulumi:pulumi:Stack: (same)
  [urn=urn:pulumi:github-prod::sigstore-github-sync::pulumi:pulumi:Stack::sigstore-github-sync-github-prod]
  ~ github:index/repository:Repository: (update) 🔒
      [id=model-transparency]
      [urn=urn:pulumi:github-prod::sigstore-github-sync::github:index/repository:Repository::model-transparency]
      [provider=urn:pulumi:github-prod::sigstore-github-sync::pulumi:providers:github::default_6_2_3::dfb01c0f-bb2a-45c2-9d92-e6152b5cee06]
    ~ pages: {
        ~ buildType: "legacy" => "workflow"
        - source   : {
            - branch: "gh-pages"
            - path  : "/"
          }
        - source   : {
            - branch: "gh-pages"
            - path  : "/"
          }
      }
@ Previewing update....
@ Previewing update....
Resources:
  ~ 1 to update
  591 unchanged