The helper suppresses output if the command runs successfully. If the
command fails, it dumps the buffered output to stdout before returning
the error. This means the happy path isn't swamped by debug noise or
xcode being intensely verbose about what kind of day it's having,
but you still get debug output when something goes wrong.
Updates tailscale/corp#9045
Signed-off-by: David Anderson <danderson@tailscale.com>
