Add REBAR to environment before executing hooks

REBAR will be set to the rebar binary which was executed and runs the
builds. Enables the use of the same binary for rebar invocations as
part of a pre or post hook like so:

    ${REBAR} escriptize

src/rebar_core.erl

@@ -496,8 +496,9 @@ run_modules([Module | Rest], Command, Config, File) ->
             {Module, Error}
     end.
 
-apply_hooks(Mode, Config, Command, Env) ->
+apply_hooks(Mode, Config, Command, Env0) ->
     Hooks = rebar_config:get_local(Config, Mode, []),
+    Env = rebar_utils:patch_env(Config, Env0),
     lists:foreach(fun apply_hook/1,
                   [{Env, Hook} || Hook <- Hooks,
                                   element(1, Hook) =:= Command orelse

src/rebar_utils.erl

@@ -53,7 +53,8 @@
          src_dirs/1,
          ebin_dir/0,
          base_dir/1,
-         processing_base_dir/1, processing_base_dir/2]).
+         processing_base_dir/1, processing_base_dir/2,
+         patch_env/2]).
 
 -include("rebar.hrl").
 
@@ -319,6 +320,23 @@ processing_base_dir(Config, Dir) ->
     AbsDir = filename:absname(Dir),
     AbsDir =:= base_dir(Config).
 
+%% @doc Returns the list of environment variables including 'REBAR' which points to the
+%% rebar executable used to execute the currently running command. The environment is
+%% not modified if rebar was invoked programmatically.
+-spec patch_env(rebar_config:config(), [{string(), string()}]) -> [{string(), string()}].
+patch_env(Config, []) ->
+    % if we reached an empty list the env did not contain the REBAR variable
+    case rebar_config:get_xconf(Config, escript, "") of
+        "" -> % rebar was invoked programmatically
+            [];
+        Path ->
+            [{"REBAR", Path}]
+    end;
+patch_env(_Config, [{"REBAR", _} | _]=All) ->
+    All;
+patch_env(Config, [E | Rest]) ->
+    [E | patch_env(Config, Rest)].
+
 %% ====================================================================
 %% Internal functions
 %% ====================================================================