task runner: use runtime.memory to avoid oversubscribing host memory

[mlin]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 1246

• 56 of 56 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 96.988%

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Totals
Change from base Build 1243:	0.02%
Covered Lines:	4476
Relevant Lines:	4615

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💛 - Coveralls

[mlin]

Might be inconvenient to have this always on, as a large value will cause serialization of test jobs on smaller machines. Maybe user control the upper-limit values for cpu and memory (defaulting to detected host values)