Allow use of caches 'read-only'

To avoid evicting useful entries, some pipeline may benefit from using existing
cache entries without writing any changes back to the cache.

Fixes #62

src/post.ts

@@ -1,12 +1,20 @@
+import * as core from '@actions/core'
+
 import * as cacheWrapper from './cache-wrapper'
 import * as cacheDependencies from './cache-dependencies'
 import * as cacheConfiguration from './cache-configuration'
 
 // Invoked by GitHub Actions
 export async function run(): Promise<void> {
+    if (isCacheReadOnly()) return
+
     await cacheWrapper.cacheWrapperDist()
     await cacheDependencies.cacheDependencies()
     await cacheConfiguration.cacheConfiguration()
 }
 
+function isCacheReadOnly(): boolean {
+    return core.getBooleanInput('cache-read-only')
+}
+
 run()