Use const initialization for INACTIVE_SPAN #514

Job	Run time
minimal-versions	57s
Doc checks	52s
fmt	15s
Cargo deny checks	40s
Clippy checks	58s
Check feature combinations (macos-latest)	54s
Check unstable	49s
Test unstable	1m 15s
Check feature combinations (ubuntu-latest)	1m 8s
Run example (dry run)	46s
Test (x86_64-linux)	2m 41s
Test (arm-android)	58s
Test (aarch64-ios)	41s
Test (aarch64-linux)	4m 39s
Test (x86_64-macos)	2m 26s
Test (arm64-android)	35s
Test (x86_64-windows)	1m 49s
Pull Request Status Checks	4s
	22m 27s

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