[release/6.0] Get richnav.yml building again (#40783)

* Get richnav.yml building again - correct build.cmd location - generally align richnav.yml with ci.yml - ignore signing, internal download args, and publish args

nit: correct name of one build step in ci.yml

* Disable pipeline in PR validation - also remove irrelevant trigger branches

* !fixup! Try doing native build w/o rich nav

nit: put `-no`s together

* !fixup! Disable rich nav explicitly - also remove installers build entirely; build will fail

nits:
- 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

Co-authored-by: Doug Bunting <6431421+dougbu@users.noreply.github.com>