Modify the dotnet/diagnostics build to enable the runtime repository to test locally-built runtimes

[jkoritzinsky]

There's a desire to add an on-demand pipeline in dotnet/runtime to run diagnostic tests (particularly SOS tests) within a runtime PR.

To do this, there's a few changes that we would need in the diagnostics repo to support this:

• A mechanism to inject additional steps before the actual build step in eng/pipelines/build.yml.
  • We could do this with an additional template parameter that's a stepList.
• A mechanism to specify using a local runtime build instead of using dotnet-install to download the runtime.

The rest of the work can be done in the runtime repository to pull in the diagnostics repo YML as necessary to author the pipeline.

cc: @max-charlamb

[tommcdon]

cc @hoyosjs @mikem8361