Increase max-old-space even if not in large runner
What does this MR do and why?
This MR is a follow-up to Increase heap size of asset compilation in CI (!168164 - merged) (and Run `cache-assets:production` jobs on `saas-lin... (!168277 - merged)) to increase max-old-space
in CI even if we’re not using the larger runner.
The default medium runner (or our gitlab-org runners) have 16 GB available, so we bump this to 8 GB.
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