don't call into CombinedOutput for subprocesses

[rickystewart]

The documentation for exec.Cmd says this about Stdout and Stderr:

// If either is an *os.File, the corresponding output from the process
// is connected directly to that file.
//
// Otherwise, during the execution of the command a separate goroutine
// reads from the process over a pipe and delivers that data to the
// corresponding Writer. In this case, Wait does not complete until the
// goroutine reaches EOF or encounters an error.

When calling CombinedOutput(), Stdout and Stderr are
bytes.Buffers and are therefore not *os.Files so they fall into
this second group. This resulted in a race condition where cancelling
the context when maxtime has passed could cause CombinedOutput() to
hang indefinitely waiting for the (finished) subprocess to "finish"
writing to its pipes.

This has been reported as an issue several times. The tracking issue is
golang/go#23019 which itself links to several
other issues that are duplicates.

To work around the issue we simply force the other behavior by creating
a temporary *os.File for the combined stdout and stderr.

[cockroach-teamcity]

This change is 

[rail]

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