oci-runtime-tools: install the test rootfs archives on x86

oci-runtime-tools: install the test rootfs archives on x86

The install recipe guarded both archives with CONFIG_x86, a symbol OpenWrt
never defines: the target symbol is CONFIG_TARGET_x86 and the architecture
symbols are CONFIG_i386 and CONFIG_x86_64. No archive was therefore packaged
and every validation test stopped at "rootfs-amd64.tar.gz: Cannot open".
Key each archive off the architecture symbol that matches it, i686 included,
so the package's own architecture guard and the archive it installs cannot
disagree.

The choice moves out of the install recipe, since a conditional inside a
define has to start at the beginning of the line to be a directive at all,
which leaves no way to indent it as the packaging guidelines ask for.

Signed-off-by: Daniel Golle <redacted>