tools: update nixpkgs-unstable to c6d65881c5624c9cae5ea6cedef24699b0c #6363

Job	Run time
Build slim tarball	1m 8s
aarch64-linux: Cache V8 build	22s
aarch64-darwin: with shared libraries	26m 42s
x86_64-linux: with shared libraries	31m 49s
x86_64-darwin: with shared libraries	41m 40s
aarch64-linux: with shared openssl-1.1.1w	25m 50s
aarch64-linux: with shared openssl-4.0.0	26m 22s
aarch64-linux: with shared openssl-3.5.5	25m 31s
aarch64-linux: with shared openssl-3.6.1	25m 50s
aarch64-linux: with shared openssl-3.0.19	26m 47s
	3h 52m 1s

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