Fixup test-bundler-parallel

* Do not chdir in the runner process, to access miniruby.  Chdir
  in worker processes instead.
* GNU make does not export newly added environment variables by
  default, set PARALLEL_TESTS_EXECUTABLE in the runner.

common.mk

@@ -1351,11 +1351,14 @@ no-test-bundler:
 
 PARALLELRSPECOPTS = --runtime-log $(srcdir)/tmp/parallel_runtime_rspec.log
 test-bundler-parallel: $(TEST_RUNNABLE)-test-bundler-parallel
-PARALLEL_TESTS_EXECUTABLE = $(XRUBY)
 yes-test-bundler-parallel: yes-test-bundler-prepare
-	$(XRUBY) -C $(srcdir) -Ispec/bundler .bundle/bin/parallel_rspec \
-		-o "--require $(srcdir)/spec/bundler/spec_helper" \
-		$(PARALLELRSPECOPTS) spec/bundler/$(BUNDLER_SPECS)
+	$(XRUBY) -I$(srcdir)/spec/bundler \
+		-e "ENV['PARALLEL_TESTS_EXECUTABLE'] = ARGV.shift" \
+		-e "load ARGV.shift" \
+		"$(XRUBY) -C $(srcdir) -Ispec/bundler .bundle/bin/rspec" \
+		$(srcdir)/.bundle/bin/parallel_rspec \
+		-o "--require spec_helper" \
+		$(PARALLELRSPECOPTS) $(srcdir)/spec/bundler/$(BUNDLER_SPECS)
 no-test-bundler-parallel:
 
 GEM = up