frontend: fix testMultiStageImplicitFrom to account for busybox changes

[thaJeztah]

It looks like there's some changes between busybox:1.34.0 and up; version 1.34.0 of the image did not have a /usr/bin directory (only /usr/sbin);

docker run --rm -it busybox:1.34.0 ls -al /usr/
total 12
drwxr-xr-x    3 root     root          4096 Sep 13  2021 .
drwxr-xr-x    1 root     root          4096 Dec 27 14:45 ..
drwxr-xr-x    2 daemon   daemon        4096 Sep 13  2021 sbin

But 1.34.1 and up do;

docker run --rm -it busybox:1.34.1 ls -al usr/
total 16
drwxr-xr-x    4 root     root          4096 Dec 21 18:28 .
drwxr-xr-x    1 root     root          4096 Dec 27 14:44 ..
drwxr-xr-x    2 root     root          4096 Dec 21 18:28 bin
drwxr-xr-x    2 daemon   daemon        4096 Dec 21 18:28 sbin

It's not immediately apparent what caused this change, or if it's in busybox itself, or in the official image only;
mirror/busybox@1_34_0...1_34_1

But either way, this change caused a test to fail:

sandbox.go:238: time="2022-12-27T13:45:25.294022820Z" level=debug msg="> creating 4gr5bno8rj7l3k7h9jxe3jhal [/bin/sh -c mkdir /usr/bin && echo -n foo > /usr/bin/go]" span="[golang 2/2] RUN mkdir /usr/bin && echo -n foo > /usr/bin/go"
sandbox.go:238: time="2022-12-27T13:45:25.433886983Z" level=debug msg="sandbox set key processing took 70.062631ms for container 5b4o358g2ryquk4s6ami38gqo"
sandbox.go:238: mkdir: can't create directory '/usr/bin': File exists

[thaJeztah]

Looks like it added a /usr/bin/env symlink;

docker run --rm -it busybox:1.34.1 ls -al usr/bin/
total 8
drwxr-xr-x    2 root     root          4096 Dec 21 18:34 .
drwxr-xr-x    4 root     root          4096 Dec 21 18:34 ..
lrwxrwxrwx    1 root     root            13 Dec 21 18:34 env -> ../../bin/env

[thaJeztah]

This one's green, @crazy-max - do you want a second review, or "good to go"?