src: fix --icu-data-dir= regression

Commit a8734af ("src: make copies of startup environment variables")
from two weeks ago introduced a regression in the capturing of the
`--icu-data-dir=` switch: it captured the string up to the `=` instead
of what comes after it.

PR-URL: nodejs#11255
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

src/node.cc

@@ -3742,7 +3742,7 @@ static void ParseArgs(int* argc,
 #endif /* HAVE_OPENSSL */
 #if defined(NODE_HAVE_I18N_SUPPORT)
     } else if (strncmp(arg, "--icu-data-dir=", 15) == 0) {
-      icu_data_dir.assign(arg, 15);
+      icu_data_dir.assign(arg + 15);
 #endif
     } else if (strcmp(arg, "--expose-internals") == 0 ||
                strcmp(arg, "--expose_internals") == 0) {

test/parallel/test-intl-no-icu-data.js

@@ -2,6 +2,8 @@
 'use strict';
 require('../common');
 const assert = require('assert');
+const config = process.binding('config');
 
 // No-op when ICU case mappings are unavailable.
 assert.strictEqual('ç'.toLocaleUpperCase('el'), 'ç');
+assert.strictEqual(config.icuDataDir, 'test/fixtures/empty/');