feat(depot-build): Add a tool to build at arbitrary tree locations

Assuming you're in `/depot/fun/clbot`, you can do this:

    tazjin@frog /d/f/clbot (master)> depot-build
    Building //fun/clbot
    /nix/store/i1zc6g58wa4819kyhaqi12zsh3hr31ph-clbot

It is automatically added to the $PATH using direnv, too.

Change-Id: Ia3341704e6317c2b8de40a3fa1be3b680d21a42d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/388
Reviewed-by: eta <eta@theta.eu.org>

tools/depot-build.nix

@@ -0,0 +1,8 @@
+# Utility script for building any arbitrary depot path in its folder.
+{ pkgs, ... }:
+
+pkgs.writeShellScriptBin "depot-build" ''
+  TARGET=$(git rev-parse --show-prefix | sed 's|/$||')
+  echo "Building //$TARGET"
+  nix-build -A $(echo $TARGET | sed 's|/|.|') $(${pkgs.git}/bin/git rev-parse --show-toplevel)
+''