unixPB: increase swap file space to 4GiB (was 2GiB)

[sxa]

Fixes adoptium/aqa-tests#6526 (or will once it is redeployed)
To deploy on an existing machine (Needs to be done on all of the test-*-s390x ones as a minimum) here is a cut and pastable list of commands:

swapoff /swapfile
dd if=/dev/zero of=/swapfile bs=65536 count=65536
chmod 600 /swapfile
mkswap /swapfile
swapon /swapfile

This should be safe unless we get machines were there is less than 4GiB free to create the swapfile in /

Checklist

• commit message has one of the standard prefixes
• faq.md updated if appropriate
• other documentation is changed or added (if applicable)
• playbook changes run through VPC or QPC (if you have access)
• VPC/QPC not applicable for this PR
• for inventory.yml changes, bastillion/nagios/jenkins updated accordingly