Consolidate exit-code parameters on runCommand in dev/bots/run_command.dart #196126

#195708 introduced checkExitCode in runCommand in dev/bots/run_command.dart, so now we have three separate exit-code parameters: checkExitCode, expectNonZeroExit, and expectedExitCode. This feels ugly and error prone. Let's migrate to something cleaner, like an enum (enum ExpectedExit { zero, nonZero, any }) or something. I filed this followup issue instead of doing it in that PR to avoid migrating callsites together with the other changes in the PR.