Drop inner `@MainActor` from `withMainSerialExecutor` #79

Job	Run time
Wasm (wasm-5.7.1-RELEASE)	43s
Windows (windows-latest, debug)	1m 49s
Linux (ubuntu-20.04, debug)	22s
Windows (windows-latest, release)	1m 21s
Linux (ubuntu-20.04, release)	51s
macOS (14.3.1, debug)	2m 1s
macOS (14.3.1, release)	1m 50s
	8m 57s

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