tools: refine `v8.nix` source definition #7483

Job	Run time
Build slim tarball	1m 10s
x86_64-linux: with shared libraries	2h 2m 12s
aarch64-darwin: with shared libraries	1h 39m 15s
x86_64-darwin: with shared libraries	2h 29m 19s
aarch64-linux: Cache V8 build	1h 3m 2s
aarch64-linux: with shared openssl-1.1.1w	22m 28s
aarch64-linux: with shared openssl-3.0.19	22m 22s
aarch64-linux: with shared openssl-4.0.0	22m 41s
aarch64-linux: with shared openssl-3.5.5	22m 45s
aarch64-linux: with shared boringssl-0.20260413.0	22m 1s
aarch64-linux: with shared openssl-3.6.1	23m 27s
	9h 30m 42s

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