Try and fix windows CI

[PragTob]

• https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/
• https://github.com/bencheeorg/benchee/runs/1866359293

[PragTob]

Wait... what a flakey :eyes:

  1) test .run_scenarios each triggers for every invocation, scenario once (Benchee.Benchmark.RunnerTest)
Error:      test/benchee/benchmark/runner_test.exs:632
     Assertion with == failed
     code:  assert global_before_sceneario_count == 1
     left:  10
     right: 1
     stacktrace:
       test/benchee/benchmark/runner_test.exs:687: anonymous fn/0 in Benchee.Benchmark.RunnerTest."test .run_scenarios each triggers for every invocation, scenario once"/1
       test/benchee/benchmark/runner_test.exs:634: (test)