Auto merge of #2368 - RalfJung:debug, r=oli-obk

Auto merge of #2368 - RalfJung:debug, r=oli-obk

Make "./miri {build,run,test}" use debug assertions but "./miri install" not

This makes `./miri run`/`./miri test` use the full set of debug assertions (including the rather expensive ones that check consistency of the Stacked Borrows cache), but `./miri install` installs a Miri *without* those debug assertions.

That's the same behavior as cargo, and helps catch Miri bugs with the test suite while making installed Miri usable for larger runs.