misc/cgo/testcarchive: log command output more consistently

Also check that executables exist immediately after building them
in parallel tests.

The parallel tests in this package occasionally fail with
"no such file or directory", implying that either the build
command failed to actually write out the binary or something
concurrently deleted it.

This is purely a shot in the dark, but I'm hoping that perhaps
the stderr output from one of these commands will shed some
light on the underlying failure mode.

For #49693

Change-Id: I2e768190c56053550879b89a3ac88c027d4741dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/369034


Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>