Avoid stack overflow in task_size bulk test

[msimberg]

stdexec in debug mode is particularly prone to triggering a stack overflow, but pika's std::execution implementation can trigger it as well. Compiling with optimizations also simply delays the stack overflow. This should fix the frequent failures on the performance tests in CI.

[codacy-production]

Coverage summary from Codacy

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Coverage variation	Diff coverage
✅ -0.04% (target: -1.00%)	✅ ∅ (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (b207b3f)	18274	13810	75.57%
Head commit (1411c10)	18274 (+0)	13803 (-7)	75.53% (-0.04%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#1332)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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