Otherwise you can do
ln -s /etc/passwd $out/foo echo "file misc $out/foo" >> $out/nix-support/hydra-build-products
and get Hydra to serve its /etc/passwd file.
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XJFHFZCA2VXGYSRDJDLAZZX3MLRH2KDT242DHWRU5XQRJ6XIHZUQC
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OOQ2D3KCLFPYNAN253PHWLBQMB6OMO2KYQWQXLTP65SQAYZWQ5LAC
FM4O2L4MNPCHWRDRHKGKSH5TFFX6R2RMCPEQ3XCECA6UVLJPLUVQC
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EDRUQ4UK643SUCYMMTS2JS4N3BHARLKRDWRH3QAYWDMWRAMPNAOQC