buildctl: do not trace unless asked.
buildctl: do not trace unless asked.

My `/tmp` was unexpectedly full of `/tmp/buildctlXXXXXXXXX`, lets stop doing
that by default.

Suggesting `/dev/null` as an example seemed a bit strange with the default
changed, so dropped that too while editing.

Signed-off-by: Ian Campbell <a8db091655a7f47ecb9c39a2d278e9662e440dc0@docker.com>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc