docker: fix OpenMPI parallel build

Change make -j ${nproc} to make -j $(nproc) in Dockerfile.cpu.
${nproc} expands to empty string (variable doesn't exist), causing
single-threaded builds. $(nproc) executes the nproc command to use
all available CPU cores.

This reduces OpenMPI build time from ~15-20 mins to ~5-8 mins on
4-core ARM64 runners.

Author: ggorman

docker/Dockerfile.cpu

@@ -45,7 +45,7 @@ RUN cd /tmp && mkdir openmpi && \
     ../configure --prefix=/opt/openmpi/ \
                  --disable-mpi-fortran \
                  --enable-mca-no-build=btl-uct --enable-mpi1-compatibility && \
-    make -j ${nproc} && \
+    make -j $(nproc) && \
     make install && \
     cd /tmp && rm -rf /tmp/openmpi