Only add ruby in $PATH, do not overwrite /usr/bin/ruby

* Fixes https://github.com/actions/setup-ruby/issues/18
* The symlink never worked on macOS, and there was a missing `await`.
* No tool should hardcode `/usr/bin/ruby`, but instead executables
  should use `#!/usr/bin/env ruby`.
* If a specific tool needs the symlink, then it can of course be
  added as part of the steps of that CI.

src/installer.ts

@@ -1,10 +1,7 @@
 import * as core from '@actions/core';
-import * as exec from '@actions/exec';
 import * as tc from '@actions/tool-cache';
 import * as path from 'path';
 
-const IS_WINDOWS = process.platform === 'win32';
-
 export async function findRubyVersion(version: string) {
   const installDir: string | null = tc.find('Ruby', version);
 
@@ -14,13 +11,5 @@ export async function findRubyVersion(version: string) {
 
   const toolPath: string = path.join(installDir, 'bin');
 
-  if (!IS_WINDOWS) {
-    // Ruby / Gem heavily use the '#!/usr/bin/ruby' to find ruby, so this task needs to
-    // replace that version of ruby so all the correct version of ruby gets selected
-    // replace the default
-    const dest: string = '/usr/bin/ruby';
-    exec.exec('sudo ln', ['-sf', path.join(toolPath, 'ruby'), dest]); // replace any existing
-  }
-
   core.addPath(toolPath);
 }