cmd/coordinator: clean up after non-VM buildlet runs
cmd/coordinator: clean up after non-VM buildlet runs

Reverse buildlets get reused. Make sure files from previous builds
don't interfere.

Fixes #10604

Change-Id: Ieab48debe1446b5fe04b8b1e3c0f9b7498c975d2
Reviewed-on: https://go-review.googlesource.com/9458
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,evandbrown/build,proppy/build,golang/build,evandbrown/build,evandbrown/build,proppy/build,proppy/build,proppy/build,golang/build,golang/build