The run in terminal request sent by exec would not work if `buildx` had
been invoked directly instead of through `docker`. If we don't find the
reexec environment variable, we use `os.Args[0]` to launch buildx
directly when invoking attach.
Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>
Invoking the shell will cause it to persist across the entire build and
to re-execute whenever the builder pauses at another location again.
This still requires using `exec` to launch the shell. Launching by frame
id is also removed since it no longer applies to this version.
Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>