Install an exact Bundler version with --conservative

Reinstalling an exact version that is already installed changes nothing, but
`gem install` removes the gem directory before extracting it again. On
self-hosted runners that share a tool cache, a concurrent job then fails at
`bundle config` with "cannot load such file -- .../bundler-X.Y.Z/lib/bundler/dsl"
(#191). With `--conservative`, RubyGems returns before touching anything when
the requirement is already met, including by a default gem. Only exact
versions pass the flag, so a `~>` constraint still gets the latest matching
release rather than the Bundler the Ruby shipped with (see #48).

`--conservative` has worked this way for `gem install` since RubyGems 1.8.23.

Author: excid3

bundler.js

@@ -144,7 +144,15 @@ export async function installBundler(bundlerVersionInput, rubygemsInputSet, lock
   const versionParts = [...bundlerVersion.matchAll(/\d+/g)].length
   const bundlerVersionConstraint = versionParts >= 3 ? bundlerVersion : `~> ${bundlerVersion}.0`
 
-  await exec.exec(gem, ['install', 'bundler', '-v', bundlerVersionConstraint])
+  const args = ['install', 'bundler', '-v', bundlerVersionConstraint]
+  // 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). A `~>` constraint still gets the
+  // latest matching release rather than whatever Bundler the Ruby shipped with.
+  if (versionParts >= 3) {
+    args.push('--conservative')
+  }
+  await exec.exec(gem, args)
 
   return bundlerVersion
 }