YAML consolidation changes part 2 (dotnet/coreclr#27649)

YAML consolidation changes part 2 (dotnet/coreclr#27649)

After finally getting a more or less green run in the runtime repo
I have prepared another batch of changes aimed at simplifying
the oncoming repo integration:

1) Use relative paths for script references;

2) Add new variable coreClrRepoRootDir with the appended path
separator so that we don't need to fork that everywhere;

3) Actually unify the first pair of build-test executions that
has become identical after the change;

4) Fixed several previously unseen repo root-relative paths;

5) Fixed missing propagation of testGroup in build-job;

6) Fixed run logs to include R2R in the name (Matt Mitchell pointed
out the other day this prevents us from analyzing both sets of logs);

7) Move job.yml to eng to enable using relative template paths;

8) Delete unused jobs.yml.

9) Move performance.yml to eng as run-performance-test.yml to
derive it from xplat-pipeline-job and avoid duplicating variables.

Thanks

Tomas

Commit migrated from https://github.com/dotnet/coreclr/commit/818b35978a1681690988b2c9c3f0b6a034d91e82