Add `merge_group:` trigger to CI workflow. #5981

Job	Run time
Bitrot check	1m 33s
Latest build on ubuntu-latest	2m 9s
Build target wasm32-wasi	53s
Latest build on windows-latest	3m 45s
Check ZFuture build on Linux	28s
Latest build on macOS-latest	2m 9s
Check ZFuture build on macOS	37s
Clippy (MSRV)	1m 17s
Clippy (beta)	11s
Check ZFuture build on Windows	3m 7s
Code coverage	59m 29s
Intra-doc links	2m 10s
Rustfmt	14s
protobuf consistency	1m 2s
Test on Linux	8m 52s
UUID validity	7s
Test Orchard on Linux	10m 56s
Test NU6 on Linux	8m 45s
Test on macOS	10m 25s
Test Orchard on macOS	15m 8s
Test on Windows	16m 59s
Test Orchard on Windows	20m 34s
Test NU6 on Windows	17m 57s
	3h 8m 47s

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