Cache validated checksums for later executions

The most common case for validation will be that the wrapper jars are unchanged
from a previous workflow run. In this case, we cache the validated wrapper
checksums to minimise the work required on a subsequent run.

Fixes #172

sources/src/configuration.ts

@@ -8,6 +8,8 @@ import path from 'path'
 
 const ACTION_ID_VAR = 'GRADLE_ACTION_ID'
 
+export const ACTION_METADATA_DIR = '.setup-gradle'
+
 export class DependencyGraphConfig {
     getDependencyGraphOption(): DependencyGraphOption {
         const val = core.getInput('dependency-graph')