mk: Don't run benchmarks with `make check`

The current suite of benchmarks for the standard distribution take a significant
amount of time to run, but it's unclear whether we're gaining any benefit from
running them. Some specific pain points:

* No one is looking at the data generated by the benchmarks. We have no graphs
  or analysis of what's happening, so all the data is largely being cast into
  the void.

* No benchmark has ever uncovered a bug, they have always run successfully.

* Benchmarks not only take a significant amount of time to run, but also take a
  significant amount of time to compile. I don't think we should mitigate this
  for now because it's useful to ensure that they do indeed still compile.

This commit disables --bench test runs by default as part of `make check`,
flipping the NO_BENCH environment variable to a PLEASE_BENCH variable which will
manually enable benchmarking. If and when a dedicated bot is set up for
benchmarking, this flag can be enabled on that bot.