[v0.10 backport] frontend: fix testMultiStageImplicitFrom to account for busybox changes

[thaJeztah]

backport of

• frontend: fix testMultiStageImplicitFrom to account for busybox changes #3436

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

(cherry picked from commit 34f9898)

[thaJeztah]

Could it be that COPY --link doesn't like it if the directory contains a symlink to something outside of the directory?

2022-12-27T17:34:44.8771369Z time="2022-12-27T17:28:31Z" level=debug msg="load cache for [integration-tests-base 11/11] LINK COPY --link --from=binaries / /usr/bin/ err: maximum timeout reached"
2022-12-27T17:34:44.8772179Z time="2022-12-27T17:28:31Z" level=debug msg="load cache for [binaries-linux 2/2] LINK COPY --link --from=buildkitd /usr/bin/buildkitd / with sha256:57d7afdfd4b9a22f8c3ac0f75d32290fe56c316a512d56ef7668aad0d7f4e148"