Add pipelineymlgen; fix 1ES officialness of validation pipelines

[dagood]

• Minimally introduce Generate build pipeline YML to avoid unclear template usage #1073
• Address https://github.com/microsoft/go-lab/issues/254 for this particular pipeline, but there are more in other repos.
  • Specifically, https://github.com/microsoft/go-lab/issues/270

Incorporates https://github.com/microsoft/go-infra/tree/main/cmd/pipelineymlgen. Not the cleanest incorporation because we can't put our tooling Go module at the root of this repository. Execution happens through pwsh eng/run.ps1 pipelineymlgen to make it work the same as other generators we currently use in this repository.

Adds reproducibility evaluation to the existing eng/_util self-test suite. Since it relies on external files, re-runs aren't reliable, so added -count 1 support to the self-test runner. This isn't a problem in CI because the runs are always fresh.

This only uses the tool to address a very specific problem with the behavior of ${{ if variables['Go1ESOfficial'] }}: in the shared template by breaking up that template while avoiding duplication. I think there's still a significant amount of low-hanging fruit in this repo's pipelines for simplification later.

Test official pipeline: https://dev.azure.com/dnceng/internal/_build/results?buildId=2831266&view=results
Test unofficial pipeline: https://dev.azure.com/dnceng/internal/_build/results?buildId=2831265&view=results

[karianna]

Pipeline seems to fail