Fix bootstrap crashes when compilers get modified

When recompiling rebar3 itself, the path swaps and cleaning removes the
modules loaded from _build/bootstrap, but the VM still manages to
discover those in _build/prod from previous builds of the escript when
building the new escript, which causes weird failures when compilers get
modified between releases.

The fix is to just clear _build/prod as part of the init of the
bootstrap script, the same way we already clear _build/bootstrap.

bootstrap

@@ -14,6 +14,11 @@ main(_) ->
     %% Clear directories for builds since bootstrapping may require
     %% a changed structure from an older one
     rm_rf("_build/bootstrap"),
+    %% When recompiling rebar3 itself, the path swaps and cleaning
+    %% removes the modules in _build/bootstrap, but the VM still
+    %% manages to discover those in _build/prod from previous builds and
+    %% cause weird failures when compilers get modified between releases.
+    rm_rf("_build/prod"),
 
     %% We fetch a few deps from hex for boostraping,
     %% so we must compile r3_safe_erl_term.xrl which