Update audience scope for OIDC token to be more specific

[ianlewis]

Update audience scope to be more specific when requesting an OIDC token.

See slsa-framework/slsa-github-generator-go#25

[ianlewis]

I'm thinking of generating a audience from the provided BuildType on the WorkflowRun. I think this matches the intent of the audience in OIDC tokens.

[ianlewis]

We can't use our build types as is because GitHub doesn't seem to like github.com in audience names so that needs to be stripped if using the buildType