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/writeExecline/default.nix

@@ -32,8 +32,7 @@ let
     else abort ''"${toString argMode}" is not a valid argMode, use one of "var", "var-full", "env", "none", "env-no-push".'';
 
 in
-  # TODO(Profpatsch): rewrite `writeScript` with `runExecline`
-  pkgs.writeScript name ''
+  depot.nix.writeScript name ''
     #!${pkgs.execline}/bin/execlineb -W${env}
     ${depot.nix.escapeExecline argList}
   ''