A quick collection of quick fixes (#20993)

* Quick fix: Simplify devBuilds.yml
  - remove an excess build step

* Quick fix: Move `SetupNugetSources` script invocations above `parameters.beforeBuild`
  - ensure NuGet.config is ready for all internal builds
  - remove now-duplicate `SetupNugetSources` invocations wherever default-build.yml is used

* Quick fix: Clean up SiteExtensions/build.cmd
  - quote all rooted paths
  - check `%ERRORLEVEL%` after every `CALL`
  - nits:
    - add a few more `ECHO` commands
    - wrap long lines

* Quick fix: Ensure `$(BuildNative)` is always set correctly
  - fix problems using `-all` or `/p:BuildAllProjects=true` without `-buildNative`
    - ensure `$(BuildNative)` is `false` where it's not supported
  - move some duplicated settings into eng/Common.props and `<Import />` the new file
  - remove now-duplicated parts of conditions using `$(BuildNative)`

* Quick fix: Consistently use `--build-*`
  - avoid `/p:Build*`  on the command line (except with eng/scripts/ci-source-build.sh)
  - nits:
    - remove now-useless `-buildNative` with `-all`
    - expand and correct a couple of related comments and messages

* Quick fix: Support `-all` together with `-projects`
  - remove need to specify `/p:BuildAllProjects=true`
  - nit: simplify some Boolean logic