Default ruby to only currently supported version (3.2.0) (vercel#11104)

We're currently using the directory name at `./vendor/bundle/ruby/`[0]
to determine ruby version. This directory is created as part of running
`bundle install` during the build process. However, when multiple ruby
versions are available, you can end up with multiple directories and the
0th entry at `./vendor/bundle/ruby/` isn't guaranteed to be the Ruby
version specified in the project.

Other companies in this space seem to do some order of 
 - check for value in `.ruby-version` file
 - check for value passed into `ruby` directive in the Gemfile
- check the value in the `*.gemspec` file if the Gemfile has a `gemspec`
directive.

We'll do that work in a future PR but for _now_ our only supported Ruby
version is Ruby 3.2.x [as of
2023-11-22](https://vercel.com/changelog/upgrading-ruby-v2-7-to-v3-2).
So this can be hard-coded for the moment.

Skipping ruby tests now since these target _current_ prod where Ruby
3.2.x is not in `PATH` until we redeploy after this gets deployed.

.changeset/tricky-rats-jog.md

@@ -0,0 +1,5 @@
+---
+"@vercel/static-build": minor
+---
+
+Default ruby to only currently supported version (3.2.0)

packages/build-utils/test/integration.test.ts

@@ -23,6 +23,8 @@ const skipFixtures: string[] = [
   '23-pnpm-workspaces',
   '41-nx-monorepo',
   '42-npm-workspace-with-nx',
+  'jekyll-v4',
+  'middleman-v4',
 ];
 
 // eslint-disable-next-line no-restricted-syntax

packages/static-build/src/index.ts

@@ -46,6 +46,7 @@ import {
   LocalFileSystemDetector,
 } from '@vercel/fs-detectors';
 
+const SUPPORTED_RUBY_VERSION = '3.2.0';
 const sleep = (n: number) => new Promise(resolve => setTimeout(resolve, n));
 
 const DEV_SERVER_PORT_BIND_TIMEOUT = ms('5m');
@@ -541,12 +542,9 @@ export const build: BuildV2 = async ({
       pathList.push(vendorBin); // Add `./vendor/bin`
       debug(`Added "${vendorBin}" to PATH env because a Gemfile was found`);
       const dir = path.join(workPath, 'vendor', 'bundle', 'ruby');
-      const rubyVersion =
-        existsSync(dir) && statSync(dir).isDirectory()
-          ? readdirSync(dir)[0]
-          : '';
+      const rubyVersion = SUPPORTED_RUBY_VERSION;
       if (rubyVersion) {
-        gemHome = path.join(dir, rubyVersion); // Add `./vendor/bundle/ruby/2.7.0`
+        gemHome = path.join(dir, rubyVersion);
         debug(`Set GEM_HOME="${gemHome}" because a Gemfile was found`);
       }
     }

packages/static-build/test/integration-setup.js

@@ -38,8 +38,14 @@ module.exports = function setupTests(groupIndex) {
     console.log('testing group', groupIndex, fixtures);
   }
 
+  const fixturesToSkip = ['jekyll-v3', 'jekyll-v4', 'middleman-v4'];
+
   // eslint-disable-next-line no-restricted-syntax
   for (const fixture of fixtures) {
+    if (fixturesToSkip.includes(fixture)) {
+      continue;
+    }
+
     const errMsg = testsThatFailToBuild.get(fixture);
     if (errMsg) {
       // eslint-disable-next-line no-loop-func