Skip reinstalling an exact Bundler version that is already installed

When Gemfile.lock names an exact version, `gem install bundler -v X.Y.Z`
reinstalls it even if it is already there: RubyGems removes the gem directory
and extracts it again. On self-hosted runners that share a tool cache, another
job loading Bundler at that moment fails with "cannot load such file --
bundler/dsl". Checking `gem list --installed` first leaves an installed
version alone; it is the same gem either way. Version ranges still install, so
`bundler: 2` and the like keep picking up new releases.

Claude-Session: https://claude.ai/code/session_01QXQDfTwJ3WAbfys8YQKrWF

Author: excid3

bundler.js

@@ -144,11 +144,24 @@ export async function installBundler(bundlerVersionInput, rubygemsInputSet, lock
   const versionParts = [...bundlerVersion.matchAll(/\d+/g)].length
   const bundlerVersionConstraint = versionParts >= 3 ? bundlerVersion : `~> ${bundlerVersion}.0`
 
+  // Reinstalling an exact version that is already there changes nothing, and `gem install`
+  // removes the gem directory before extracting it again, which breaks the other jobs
+  // sharing a tool cache on self-hosted runners (#191)
+  if (versionParts >= 3 && await isGemInstalled(gem, 'bundler', bundlerVersion)) {
+    console.log(`Bundler ${bundlerVersion} is already installed`)
+    return bundlerVersion
+  }
+
   await exec.exec(gem, ['install', 'bundler', '-v', bundlerVersionConstraint])
 
   return bundlerVersion
 }
 
+async function isGemInstalled(gem, name, version) {
+  const { exitCode } = await exec.getExecOutput(gem, ['list', '--installed', `^${name}$`, '-v', version], { ignoreReturnCode: true, silent: true })
+  return exitCode === 0
+}
+
 function bundlerConfigSetArgs(bundlerVersion, key, value) {
   if (bundlerVersion.startsWith('1')) {
     return ['config', '--local', key, value]