Build linux-amd64 binary in installer-artifacts

This will be used by oc in the next release to allow extracting an
x86_64 installer from another architecture's payload.

Due to the size of the installer binary, building each in a separate
stage minimizes resource requirements when using imagebuilder.  This
does require a matching change to ocp-build-data to add a second builder
stage.

images/installer-artifacts/Dockerfile.rhel

@@ -1,11 +1,18 @@
 # This Dockerfile builds an image containing the Mac version of the installer layered
 # on top of the Linux installer image.
 
-FROM registry.ci.openshift.org/ocp/builder:rhel-8-golang-1.15-openshift-4.8 AS builder
+FROM registry.ci.openshift.org/ocp/builder:rhel-8-golang-1.16-openshift-4.8 AS macbuilder
 WORKDIR /go/src/github.com/openshift/installer
 COPY . .
 RUN go generate ./data && \
     SKIP_GENERATION=y GOOS=darwin GOARCH=amd64 hack/build.sh
 
+FROM registry.ci.openshift.org/ocp/builder:rhel-8-golang-1.16-openshift-4.8 AS linuxbuilder
+WORKDIR /go/src/github.com/openshift/installer
+COPY . .
+RUN go generate ./data && \
+    SKIP_GENERATION=y GOOS=linux GOARCH=amd64 hack/build.sh
+
 FROM registry.ci.openshift.org/ocp/4.8:installer
-COPY --from=builder /go/src/github.com/openshift/installer/bin/openshift-install /usr/share/openshift/mac/openshift-install
+COPY --from=macbuilder /go/src/github.com/openshift/installer/_output/bin/darwin_amd64/openshift-install /usr/share/openshift/mac/openshift-install
+COPY --from=linuxbuilder /go/src/github.com/openshift/installer/_output/bin/linux_amd64/openshift-install /usr/share/openshift/linux_amd64/openshift-install