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/binify/default.nix

@@ -10,7 +10,7 @@
 # with `binify { exe = …; name = "hello" }`.
 { exe, name }:
 
-pkgs.runCommandLocal "binify-${name}" {} ''
+pkgs.runCommandLocal "${name}-bin" {} ''
   mkdir -p $out/bin
   ln -sT ${lib.escapeShellArg exe} $out/bin/${lib.escapeShellArg name}
 ''