feat(nix/runExecline): add runExecline

runExecline is a primitive that just does not care.

It’s similar to `runCommand`, but instead of concatenating bash
scripts left and right, it actually *uses* the features of
`derivation`, passing things to `args` and making it possible to
overwrite the `builder` in a sensible manner.

Additionally, it provides a way to pass a nix string to `stdin` of the
build script.

Similar to `writeExecline`, the passed script is not a string, but a
nested list of nix lists representing execline blocks. Escaping is
done by the implementation, the user can just use normal nix strings.

Change-Id: I890d9e5d921207751cdc8cc4309381395d92742f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/701
Reviewed-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

nix/runExecline/default.nix

@@ -0,0 +1,19 @@
+{ depot, pkgs, lib, ... }:
+let
+  runExecline = import ./runExecline.nix {
+    inherit (pkgs) stdenv;
+    inherit (depot.nix) escapeExecline getBins;
+    inherit pkgs lib;
+  };
+
+  tests = import ./tests.nix {
+    inherit runExecline;
+    inherit (depot.nix) getBins;
+    inherit (pkgs) stdenv coreutils;
+    inherit pkgs;
+  };
+
+in {
+  __functor = _: runExecline;
+  inherit tests;
+}