cmd/dist: require absolute path to executable in flattenCommandLine

cmd/dist: require absolute path to executable in flattenCommandLine

This should help to prevent bugs from unintended use of system tools,
especially the system or bootstrap "go" command.
(Suggested by Austin on CL 452678.)

Updates #31567.

Change-Id: I71809ee30d06eda4b5ff8f90656d4f1a462d35dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/452776
Reviewed-by: Austin Clements <austin@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>