ci: disabled: riscv: minimise docker overlays

Suggested by @bjorn3

Every RUN command creates a new overlay on top of the image as of before
the RUN command. Using fewer RUN commands prevents intermediate overlays
(which in this case would have contained the entire Linux source tree).

Author: tblah

src/ci/docker/host-x86_64/disabled/riscv64gc-linux/Dockerfile

@@ -40,9 +40,9 @@ RUN curl https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.6.16.tar.xz | tar
     cp linux.config linux-5.6.16/.config && \
     cd /build/linux-5.6.16 && \
     make olddefconfig && \
-    make -j$(nproc) vmlinux
-RUN cp linux-5.6.16/vmlinux /tmp
-RUN rm -rf linux-5.6.16
+    make -j$(nproc) vmlinux && \
+    cp vmlinux /tmp && \
+    rm -rf linux-5.6.16
 
 # Compile an instance of busybox as this provides a lightweight system and init
 # binary which we will boot into. Only trick here is configuring busybox to