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/writeScriptBin/default.nix

@@ -0,0 +1,12 @@
+{ depot, ... }:
+
+# Like writeScript,
+# but put the script into `$out/bin/${name}`.
+
+name:
+script:
+
+depot.nix.binify {
+  exe = (depot.nix.writeScript name script);
+  inherit name;
+}