BridgeJS: Use let for non-mutated __bjs_self in generated toJSObject() #1991

Job	Run time
check-bridgejs-generated	2m 52s
Build for native target (macos-15, Xcode_16.4)	4m 6s
format	34s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	17m 28s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	13m 53s
Build and Test (ubuntu-24.04, https://download.swift.org/swift-6.3-branch/ubuntu2404/swift-6.3-DE...	13m 46s
Build and Test (ubuntu-24.04, https://download.swift.org/development/ubuntu2404/swift-DEVELOPMENT...	13m 14s
build-examples	43m 37s
deploy-examples	-1s
	1h 49m 29s

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