[ffigen] Retain listener blocks before invocation (#1574) #1

Job	Run time
build (ubuntu, stable, native_assets_builder, false)	1m 4s
build (ubuntu, stable, native_assets_cli, false)	50s
build (ubuntu, stable, native_toolchain_c, false)	2m 20s
build (ubuntu, dev, native_assets_builder, false)	1m 7s
build (ubuntu, dev, native_assets_cli, false)	57s
build (ubuntu, dev, native_toolchain_c, false)	1m 54s
build (macos, stable, native_assets_builder, false)	40s
build (macos, stable, native_assets_cli, false)	37s
build (macos, stable, native_toolchain_c, false)	2m 42s
build (macos, dev, native_assets_builder, false)	44s
build (macos, dev, native_assets_cli, false)	3m 25s
build (macos, dev, native_toolchain_c, false)	2m 29s
build (windows, stable, native_assets_builder, false)	2m 25s
build (windows, stable, native_assets_cli, false)	1m 29s
build (windows, stable, native_toolchain_c, false)	4m 17s
build (windows, dev, native_assets_builder, false)	2m 25s
build (windows, dev, native_assets_cli, false)	1m 52s
build (windows, dev, native_toolchain_c, false)	5m 3s
coverage-finished	0s
	36m 20s

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