[CI] Configure Mergify to use chiselbot as bot account (backport #3770) #6587

Job	Run time
Standard Library Tests (2.13.10)	1m 25s
Integration Tests (w/ chiseltest)	3m 25s
Standard Library Tests (2.12.17)	1m 32s
ci (ubuntu-20.04, 8, 2.13.10, 2.4, firtool-1.37.0)	17m 39s
Formatting	55s
Test Scala-CLI Template	2m 12s
ci (ubuntu-20.04, 8, 2.12.17, 2.4, firtool-1.37.0)	17m 30s
Build Mdoc & Website	4m 49s
check tests	1s
all tests passed	1s
publish	0s
	49m 29s

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