Bump nix from 0.26.2 to 0.27.1 in /src/agent #1373

Job	Run time
afl	23s
azcopy	18s
aflpp	2m 2s
agent (self-hosted, 1ES.Pool=onefuzz-ci, 1ES.ImageOverride=github-runner-image-windows-2019)	1s
agent (self-hosted, 1ES.Pool=onefuzz-ci, 1ES.ImageOverride=github-runner-image-ubuntu-20.04)	1s
agent (self-hosted, 1ES.Pool=onefuzz-ci-arm64, 1ES.ImageOverride=github-runner-image-ubuntu-20.04...	1s
Check Bicep files	51s
build-integration-tests-linux	3m 42s
check-pr	36s
build-integration-tests-windows	2m 7s
contrib-webhook-teams-service	32s
dotnet-fuzzing-tools-windows	2m 25s
onefuzztypes	39s
dotnet-fuzzing-tools-linux	1m 0s
proxy	20m 34s
python-safety	33s
radamsa-linux	1m 3s
radamsa-win64	1m 53s
service	2m 39s
cli	4m 24s
package	0s
integration-tests-linux	0s
	45m 44s

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