feat(nix/writeExecline): add writeExecline

This is a writer, similar to `pkgs.writeBashScript` or
`pkgs.writers.writePython3`.

The difference is that we can correctly write all execline scripts by
using nix lists of lists, so the user doesn’t have to care about
escaping arguments (like they have to in bash scripts with
`lib.escapeShellArg` for example).

Change-Id: I2f2874cf61170ddca07b89b692f762725f4a75dc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/625
Reviewed-by: Kane York <rikingcoding@gmail.com>
Reviewed-by: tazjin <mail@tazj.in>

third_party/default.nix

@@ -59,6 +59,7 @@ let
       cudatoolkit
       darwin
       dockerTools
+      execline
       fetchFromGitHub
       fetchgit
       fetchurl
@@ -130,6 +131,7 @@ let
       thttpd
       tree
       which
+      writeScript
       writeShellScript
       writeShellScriptBin
       writeText