tools: refine `v8.nix` source definition #7482

Job	Run time
Build slim tarball	1m 11s
aarch64-linux: Cache V8 build	1h 3m 31s
x86_64-linux: with shared libraries	1h 51m 42s
x86_64-darwin: with shared libraries	2h 15m 38s
aarch64-darwin: with shared libraries	1h 54m 3s
aarch64-linux: with shared openssl-3.6.2	21m 25s
aarch64-linux: with shared boringssl-0.20260508.0	21m 50s
aarch64-linux: with shared openssl-1.1.1w	21m 19s
aarch64-linux: with shared openssl-3.0.20	22m 1s
aarch64-linux: with shared openssl-3.5.6	21m 53s
aarch64-linux: with shared openssl-4.0.0	21m 26s
	9h 15m 59s

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