CI: give aarch64-linux a maxrss parameter

to avoid getting OOM killed. We need to have a high number of GitHub
Actions runners simultaneously active, otherwise the hardware ends up
underutilized. But with zig build running in parallel now, this
sometimes results in memory usage spikes that have been causing the
runner to get killed.

This number is the total physical memory (251G) divided by the number of
runners we have active (11).

See related commit 2d41dac57d which did
the same but for our x86_64-linux machines.

ci/aarch64-linux-debug.sh

@@ -60,6 +60,7 @@ stage3-debug/bin/zig build -Dtarget=arm-linux-musleabihf
 # TODO: add -fqemu back to this line
 
 stage3-debug/bin/zig build test docs \
+  --maxrss 24696061952 \
   -fwasmtime \
   -Dstatic-llvm \
   -Dtarget=native-native-musl \