nix/pkgs/pomerium: move envoy zip appending to postFixup phase

This ensures that we don't immediately remove it (because strip will otherwise
just strip it because it's trailing content).

nix/pkgs/pomerium.nix

@@ -52,8 +52,11 @@ buildGoModule rec {
   '';
 
   installPhase = ''
-    mkdir -p $out/bin
-    cat $GOPATH/bin/pomerium $NIX_BUILD_TOP/envoy/envoy.zip >$out/bin/pomerium
+    install -Dm0755 $GOPATH/bin/pomerium $out/bin/pomerium
+  '';
+  postFixup = ''
+    cat $out/bin/pomerium $NIX_BUILD_TOP/envoy/envoy.zip >$out/bin/pomerium.new
+    mv $out/bin/pomerium.new $out/bin/pomerium
     chmod +x $out/bin/pomerium
     zip --adjust-sfx $out/bin/pomerium
   '';