build: Disable completion auto-init in Nix flake

The flake correctly installs lazy-loaded shell completions,
so we don't need to manually load them as part of `kubesel init`.

Author: eth-p

flake.nix

@@ -44,6 +44,7 @@
               mkdir -p $out/bin
               go build -o $out/bin/kubesel \
                 -ldflags '-X main.GIT_REVISION=${self.rev or self.dirtyRev or "unknown"}' \
+                -tags no_init_completions \
                 ./
 
               echo "generating fish completions"