cmd/bent: clean up filename mangling

Move away from direct string manipulation, to using path.Join.

This should not be a functional change.

Change-Id: I320756c164fb0300904c934067a2f3084978ad38
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/290991
Trust: Jeremy Faller <jeremy@golang.org>
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
2 files changed