support/testing: test_openssh: use Bootlin stable toolchain
The commit backported by [1] enabled the Bootlin bleeding-edge
toolchain providing 5.4 kernel headers but the openssh test
still use a 4.19 prebuilt kernel. Indeed the prebuilt kernel
has been updated to 5.10 in Buildroot 2021.08 [2].
Fixes:
https://gitlab.com/kubu93/buildroot/-/jobs/1780867274
[1] 8d4939c9e0342d8b99d863fbdcaf935d42038469
[2] https://git.buildroot.net/buildroot/commit/?id=3cf2782906d5cc05a3966c25518d2ce01f11afc8
Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>