refactor(nix): use our own writeScript(Bin)

We have this nice `runExecline` now, so we don’t need to use
`runCommand` (which spawns bash) just to write a simple script.

Change-Id: I2941ed8c1448fa1d7cc02dc18b24a8a945b2c38b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/704
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: BuildkiteCI

nix/runExecline/tests.nix

@@ -1,4 +1,4 @@
-{ stdenv, pkgs, runExecline, getBins
+{ stdenv, pkgs, runExecline, getBins, writeScript
 # https://www.mail-archive.com/skaware@list.skarnet.org/msg01256.html
 , coreutils }:
 
@@ -19,23 +19,6 @@ let
            "s6-test"
          ];
 
-  # lol
-  writeScript = name: script: runExecline name {
-    derivationArgs = {
-      inherit script;
-      passAsFile = [ "script" ];
-      preferLocalBuild = true;
-      allowSubstitutes = false;
-    };
-  } [
-      "importas" "-ui" "s" "scriptPath"
-      "importas" "-ui" "out" "out"
-      "foreground" [
-        bins.mv "$s" "$out"
-      ]
-      bins.s6-chmod "0755" "$out"
-  ];
-
   # execline block of depth 1
   block = args: builtins.map (arg: " ${arg}") args ++ [ "" ];