Only runs a part of targets when run dist release in PRs

I have 14 targets to build using cargo-dist, but it is too expensive to verify build them all in PRs. I would like to only build 3 of them to allow a minimal verification and for most people who want to get artifacts from PR's GitHub actions:
- x86 linux
- x86 windows
- apple darwin

Start by tracing how cargo-dist configures the dist release GitHub Actions workflow for pull requests and how its target list is selected. The change is done when PR verification builds only x86 Linux, x86 Windows, and Apple Darwin artifacts instead of all 14 targets.

Written by the indexing model from the issue text.