Detect self-hosted runners not matching a GitHub-hosted runner image

* And give instructions for that case.
* Get the OS and OS version ourselves since some users set ImageOS incorrectly.
* This partially reverts commit ad1ebae995.

ruby-builder.js

@@ -1,6 +1,7 @@
 const os = require('os')
 const fs = require('fs')
 const path = require('path')
+const core = require('@actions/core')
 const exec = require('@actions/exec')
 const io = require('@actions/io')
 const tc = require('@actions/tool-cache')
@@ -13,10 +14,6 @@ const releasesURL = 'https://github.com/ruby/ruby-builder/releases'
 const windows = common.windows
 
 export function getAvailableVersions(platform, engine) {
-  if (!common.supportedPlatforms.includes(platform)) {
-    throw new Error(`Unsupported platform ${platform}`)
-  }
-
   return rubyBuilderVersions[engine]
 }
 
@@ -27,7 +24,21 @@ export async function install(platform, engine, version) {
     if (inToolCache) {
       rubyPrefix = inToolCache
     } else {
-      rubyPrefix = common.getToolCacheRubyPrefix(platform, version)
+      const toolCacheRubyPrefix = common.getToolCacheRubyPrefix(platform, engine, version)
+      if (common.isSelfHostedRunner()) {
+        const rubyBuildDefinition = engine === 'ruby' ? version : `${engine}-${version}`
+        core.error(
+          `The current runner (${common.getOSNameVersionArch()}) was detected as self-hosted and not matching a GitHub-hosted runner image.\n` +
+          `In such a case, you should install Ruby in the $RUNNER_TOOL_CACHE yourself, for example using https://github.com/rbenv/ruby-build:\n` +
+          `You can take inspiration from this workflow for more details: https://github.com/ruby/ruby-builder/blob/master/.github/workflows/build.yml\n` +
+          `$ ruby-build ${rubyBuildDefinition} ${toolCacheRubyPrefix}\n` +
+          `Once that completes successfully, mark it as complete with:\n` +
+          `$ touch ${common.toolCacheCompleteFile(toolCacheRubyPrefix)}\n` +
+          `It is your responsibility to ensure installing Ruby like that is not done in parallel.\n`)
+        process.exit(1)
+      } else {
+        rubyPrefix = toolCacheRubyPrefix
+      }
     }
   } else if (windows) {
     rubyPrefix = path.join(`${common.drive}:`, `${engine}-${version}`)