Get richnav.yml building again (#40729)

- correct build.cmd location
- generally, align richnav.yml with ci.yml
  - ignore signing, internal download args, and publish args
  - perform native build w/o rich nav and don't do installers build; both fail
- disable pipeline in PR validation
  - also remove irrelevant trigger branches

nits:
- correct name of one build step in ci.yml
- don't bother setting `$(EnableRichCodeNavigation)` or `$(OnlyPackPlatformSpecificPackages)` to `true`
  - job.yml sets `$(EnableRichCodeNavigation)` variable and `msbuild` will find that in environment
  - `$(OnlyPackPlatformSpecificPackages)` is not relevant in this pipeline; not packing