Fix os.getenv of lua on Windows

Change to use os_getenv instead of getenv because environment variable
set by uv_os_setenv can not be get with getenv.

test/functional/lua/overrides_spec.lua

@@ -300,3 +300,19 @@ describe('package.path/package.cpath', function()
     eq(new_paths_str, eval_lua('package.path'):sub(1, #new_paths_str))
   end)
 end)
+
+describe('os.getenv', function()
+  it('returns nothing for not set env var', function()
+    eq(NIL, funcs.luaeval('os.getenv("XTEST_1")'))
+  end)
+  it('returns env var set by the parent process', function()
+    local  value = 'foo'
+    clear({env = {['XTEST_1']=value}})
+    eq(value, funcs.luaeval('os.getenv("XTEST_1")'))
+  end)
+  it('returns env var set by let', function()
+    local value = 'foo'
+    meths.command('let $XTEST_1 = "'..value..'"')
+    eq(value, funcs.luaeval('os.getenv("XTEST_1")'))
+  end)
+end)