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>

bin/__dispatch.sh

@@ -47,6 +47,9 @@ case "${TARGET_TOOL}" in
   git-bug)
     attr="third_party.git-bug"
     ;;
+  depot-build)
+    attr="tools.depot-build"
+    ;;
   *)
     echo "The tool '${TARGET_TOOL}' is currently not installed in this repository."
     exit 1

bin/depot-build

@@ -0,0 +1 @@
+__dispatch.sh