Compile R packages with less parallelism: OpenMx was OOM-killed

The retry loop converged 6 -> 10 -> 10 bundles and stalled. The cause was
not a race after all:

  g++: fatal error: Killed signal terminated program cc1plus

OpenMx instantiates enough Eigen templates that a single cc1plus needs
several GB, and at -j5 on an 8GB runner the kernel killed it every time -
which is why both retries stopped at exactly 10/22 rather than creeping
forward. That in turn blocked jaspFactor, jaspSem, mxsem and semPlot.

R packages now compile at -j2 (R_JOBS) independently of the -j used for
the C++ build, and the runner VM was given 16GB.

build/build-jasp.sh

@@ -260,6 +260,15 @@ PY
 # Retrying is the cheap fix: renv skips what is already installed, so each pass
 # starts from more complete state and converges. Passes are capped so a genuine
 # failure cannot loop forever.
+# OpenMx instantiates enough Eigen templates that a single cc1plus can take
+# several GB. At the top-level -j the compiler gets OOM-killed
+# ("g++: fatal error: Killed signal terminated program cc1plus"), which is not
+# a race — it stalls at the same module count on every retry. Compile R packages
+# with less parallelism than the C++ build uses.
+R_JOBS="${R_JOBS:-2}"
+export MAKEFLAGS="-j${R_JOBS}"
+log "compiling R packages with -j${R_JOBS} (OpenMx needs the headroom)"
+
 EXPECTED_MODULES=22
 for attempt in 1 2 3; do
   log "module build pass ${attempt}/3"