[rush] Fix an issue where "rushx" did not pass additional parameters to the shell script

[octogonz]

Fixes #1232

Consider this script:

package.json

   . . .
  "scripts": {
    "build": "rm -f lib && node ./build.js"
  }
   . . .

Suppose that you invoke this command:

$ rushx build a b c --production

BEFORE this PR, the shell would execute: rm -f lib && node ./build.js

AFTER this PR, the shell will execute: rm -f lib && node ./build.js a b c --production

Ideally we want a different design, where rushx has full syntax validation and --help for every parameter. But until that ideal is realized, this PR lets people pass subcommands to their shell scripts similar to how NPM and Yarn do.