tools: update nixpkgs-unstable to c6d65881c5624c9cae5ea6cedef24699b0c #6292

Job	Run time
Build slim tarball	1m 10s
aarch64-linux: Cache V8 build	22s
aarch64-darwin: with shared libraries	26m 44s
x86_64-darwin: with shared libraries	43m 7s
x86_64-linux: with shared libraries	32m 12s
aarch64-linux: with shared openssl-4.0.0	26m 10s
aarch64-linux: with shared openssl-3.0.19	26m 12s
aarch64-linux: with shared openssl-3.5.5	26m 14s
aarch64-linux: with shared openssl-1.1.1w	25m 35s
aarch64-linux: with shared openssl-3.6.1	26m 4s
	3h 53m 50s

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