mgmt, improve performance on automation from specs "SDK Validation" CI

We currently generate SDK twice, to avoid us the effect to parse tspconfig.yaml and figure out which folder would be used to generate SDK

However, TCGC is pretty slow in big TypeSpec. So in extreme case, we double the time from 35min to 1:15 hour.
https://dev.azure.com/azure-sdk/public/_build/results?buildId=5778740&view=results

Not urgent. TCGC likely going to improve on their side as well.