IMPROVEMENT: skip doing a new build if a build has already been done

I noticed in the docs of [RunExtraTests]:

`"If C<RunExtraTests> is listed after one of the normal test-running
plugins (e.g. C<MakeMaker> or C<ModuleBuild>), then the dist will not
be rebuilt between running the normal tests and the extra tests."`

and I thought "no, it cannot be so!" -- but indeed, RunExtraTests's 'build' sub checks for -d blib, but [MakeMaker::TestRunner] does not, so if it comes after, it does another build!

I thought about comparing the mtime of Makefile.PL to Build.PL, but TestRunners are invoked only after a build has completed, so I can't think of any non-crazy case where the file would get modified and we'd want to re-run it.