flake: drop unnecessary fileContents binding

Since the tailscale derivation already has a `pkgs` binding, we can
use `pkgs.lib`. Alternatively, we could have used `nixpkgs.lib`, as
`fileContents` doesn't need a system to use (anymore?).

Signed-off-by: Cole Helbling <cole.helbling@determinate.systems>

flake.nix

@@ -41,14 +41,6 @@
   };
 
   outputs = { self, nixpkgs, flake-utils, flake-compat }: let
-    # Grab a helper func out of the Nix language libraries. Annoyingly
-    # these are only accessible through legacyPackages right now,
-    # which forces us to indirect through a platform-specific
-    # path. The x86_64-linux in here doesn't really matter, since all
-    # we're grabbing is a pure Nix string manipulation function that
-    # doesn't build any software.
-    fileContents = nixpkgs.legacyPackages.x86_64-linux.lib.fileContents;
-
     # tailscaleRev is the git commit at which this flake was imported,
     # or the empty string when building from a local checkout of the
     # tailscale repo.
@@ -74,7 +66,7 @@
       name = "tailscale";
 
       src = ./.;
-      vendorSha256 = fileContents ./go.mod.sri;
+      vendorSha256 = pkgs.lib.fileContents ./go.mod.sri;
       nativeBuildInputs = pkgs.lib.optionals pkgs.stdenv.isLinux [ pkgs.makeWrapper pkgs.git ];
       ldflags = ["-X tailscale.com/version.GitCommit=${tailscaleRev}"];
       CGO_ENABLED = 0;