ci: set correct arch for rootfs tests

With the commit 01e5cfc "CI: Add target/arch tags (no suffix) for
snapshot images"[1] the os/platform is set for all images, which is usually
different from what the GitHub action runner uses (x86). The Docker
deamon still tries to fetch the x86 version and fails.

This commit explicitly sets the fitting arch.

[1]: https://github.com/openwrt/docker/commit/01e5cfccd73a72ecab730496607c7c22b904f366



Signed-off-by: Paul Spooren <mail@aparcar.org>
(cherry picked from commit d359fa04eda29638b9326c194490685c1177fd49)