Use Build Matrix Trimming in (at least) nightly PR validation

In PRs that only change a couple of images, we waste a ton of compute running build legs that ultimately do nothing. In the nightly repo, I think we can better optimize for PR validation speed by turning on [Build matrix trimming](https://github.com/dotnet/docker-tools/pull/1449).

Open question: should we should turn it on for any other pipelines?

**Definitely:**
- nightly PR validation

**Maybe:**
- nightly (no-cache) PR validation
- main PR validation
- main (no-cache) PR validation
- release PR validation
- etc.