Limit the number of threads for subprocess tests #1135

Job	Run time
Tests / vale	12s
Tests / cargo-deny	34s
Tests / cargo-fmt	18s
Tests / cargo-clippy	41s
Tests / Linux AMD64	1m 27s
Tests / FreeBSD AMD64	3m 30s
Tests / macOS AMD64	5m 44s
Tests / macOS ARM64	1m 24s
Tests / inko-fmt	53s
Nightly container	3m 6s
Standard library documentation	1m 40s
Documentation	24s
	19m 53s

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