test: the cross-compile check never looks at the multus build stage #4500

The cross-compile check added in #4498 skips the multus build stage completely. hack/image-builders-cross-compile.bats treats a Dockerfile stage as compiling when a RUN command is go build/test/install, make, gradle, a node package manager, or a script matching *build.sh. The multus-cni builder runs upstream ./hack/build-go.sh. That name does not match [^[:space:]]*build[.]sh, so the stage counts as not compiling and nothing checks it.

Multus itself builds correctly today. Its builder is on $BUILDPLATFORM, and upstream build-go.sh takes the target from TARGETPLATFORM. The problem is only that the check says it covers this stage and it does not.

Changing the pattern to [^[:space:]]*build[^[:space:]/]*[.]sh would pull the stage in. It would then pass because of the TARGETARCH in the curl step of the same stage. That is the per-stage limit the test header already names: the check looks at the whole stage, not at the line that compiles.