Bump nix from 0.26.2 to 0.27.1 in /src/agent #1778

Job	Run time
contrib-webhook-teams-service	48s
check-pr	2m 18s
agent (self-hosted, 1ES.Pool=onefuzz-ci, 1ES.ImageOverride=github-runner-image-windows-2019)	1s
onefuzztypes	47s
azcopy	16s
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
python-safety	30s
proxy	23m 27s
afl	6s
aflpp	36s
Check Bicep files	37s
build-integration-tests-linux	10s
build-integration-tests-windows	44s
dotnet-fuzzing-tools-linux	1m 24s
dotnet-fuzzing-tools-windows	5m 31s
radamsa-linux	6s
radamsa-win64	22s
service	2m 16s
cli	4m 13s
integration-tests-linux	50s
package	0s
	45m 4s

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