cmd/bench: clean build cache in between bent and sweet

Currently /workdir/gocache uses up 9.3 GiB of 16 GiB of available
space in /workdir, causing Sweet to not have enough room to copy
tile38's assets over by the time we get to that.

Thing is, in these benchmarks we basically don't care about the build
cache at all since we only build everything once. Clean the cache at
least once in between bent and Sweet to let Sweet complete.

Fixes golang/go#58765.

Change-Id: I6f10a7eee0218ca2d2b0358a11cfd41af00b0667
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/472235
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed