Update `shell.nix` to use LLVM 18 from `nixpkgs-unstable` #11386

Job	Run time
x86_64-musl-test	38m 47s
check_format	1m 53s
x86_64-gnu-test-preview_mt	6m 0s
x86_64-gnu-test (1.7.3)	36m 20s
x86_64-gnu-test (1.8.2)	33m 53s
deploy_api_docs	0s
x86_64-gnu-test (1.9.2)	35m 34s
x86_64-gnu-test (1.10.1)	34m 53s
x86_64-gnu-test (1.11.2)	33m 41s
x86_64-gnu-test (1.12.1)	35m 40s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	32m 42s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	36m 2s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	36m 28s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	35m 32s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	35m 38s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	34m 21s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	33m 45s
	8h 21m 9s

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