fix(obs): Require at least 8GB RAM for building the Rust package in OBS

[lslezak]

Problem

• The build on PPC or S390 might fail in OBS because of insufficient RAM in the worker

Solution

• Require at least 8GB RAM in the worker
• Additionally require at least 4 parallel jobs (the build on S390 takes ages with just 2 jobs...)

Notes

• Inspired by this example file
• More details in the OBS documentation
• Let's see how it helps, we might fine tune the config later again...