Add a self-hosted input to force considering the current runner as se…

…lf-hosted

* Since checking for self-hosted is based on guessing as there is
  no proper predicate for it in GitHub Actions (AFAIK).

action.yml

@@ -30,6 +30,11 @@ inputs:
     description: |
       Arbitrary string that will be added to the cache key of the bundler cache. Set or change it if you need
       to invalidate the cache.
+  self-hosted:
+    description: |
+      Consider the runner as a self-hosted runner, which means not using prebuilt Ruby binaries which only work
+      on GitHub-hosted runners or self-hosted runners with a very similar image to the ones used by GitHub runners.
+      The default is to detect this automatically based on the OS, OS version and $RUNNER_TOOL_CACHE.
 outputs:
   ruby-prefix:
     description: 'The prefix of the installed ruby'

common.js

@@ -13,6 +13,10 @@ export const windows = (os.platform() === 'win32')
 // Extract to SSD on Windows, see https://github.com/ruby/setup-ruby/pull/14
 export const drive = (windows ? (process.env['GITHUB_WORKSPACE'] || 'C')[0] : undefined)
 
+export const inputs = {
+  selfHosted: undefined
+}
+
 export function partition(string, separator) {
   const i = string.indexOf(separator)
   if (i === -1) {
@@ -166,8 +170,13 @@ const GitHubHostedPlatforms = [
 // * the OS and OS version does not correspond to a GitHub-hosted runner image,
 // * or the hosted tool cache is different from the default tool cache path
 export function isSelfHostedRunner() {
-  return !GitHubHostedPlatforms.includes(getOSNameVersionArch()) ||
-      getRunnerToolCache() !== getDefaultToolCachePath()
+  if (inputs.selfHosted === undefined) {
+    throw new Error('inputs.selfHosted should have been already set')
+  }
+
+  return inputs.selfHosted === 'true' ||
+    !GitHubHostedPlatforms.includes(getOSNameVersionArch()) ||
+    getRunnerToolCache() !== getDefaultToolCachePath()
 }
 
 let virtualEnvironmentName = undefined

index.js

@@ -16,6 +16,7 @@ const inputDefaults = {
   'bundler-cache': 'false',
   'working-directory': '.',
   'cache-version': bundler.DEFAULT_CACHE_VERSION,
+  'self-hosted': 'false',
 }
 
 // entry point when this action is run on its own
@@ -39,6 +40,7 @@ export async function setupRuby(options = {}) {
       inputs[key] = core.getInput(key) || inputDefaults[key]
     }
   }
+  common.inputs.selfHosted = inputs['self-hosted']
 
   process.chdir(inputs['working-directory'])