Use _single_run to perform the prefetch run

[joeleba]

We've been (sort of) breaking abstraction by performing bazel.command directly in the _run_benchmark method. This change would improve the overall code readability.

Changes:

• Use _single_run to perform the prefetch run
• Change the signature of _single_run to accept args instead of separating expressions and options.
• Minor: Change "build env" to "build event" in a log entry.