CI: give x86_64-linux a maxrss parameter

to avoid getting OOM killed. It's been happening regularly recently. 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 (126G) divided by the number of
runners we have active (6).

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