release.nix: refactor with fileContents

nixos/release.nix

···
 
 let
 
-  version = builtins.readFile ../.version;
+  version = fileContents ../.version;
   versionSuffix =
     (if stableBranch then "." else "pre") + "${toString nixpkgs.revCount}.${nixpkgs.shortRev}";