oci-runtime-tools: build runtimetest statically

oci-runtime-tools: build runtimetest statically

runtimetest is copied into the validation bundle, whose rootfs carries a
static busybox and no dynamic loader, so the dynamically linked binary the
generic Go recipe produces fails to execute and every validation test that
inspects the container from the inside aborts.

Build that one command again as the static binary upstream's own runtimetest
target produces. Upstream gets there by asking the external linker for a
static link, while disabling cgo reaches the same result without fighting the
-linkmode external that the generic Go recipe would otherwise impose.

Signed-off-by: Daniel Golle <redacted>