Add skip-save feature

The skip-save option allows users to force the cache to skip saving the cache.  skip-save can be set to a constant or to an expression that will be evaluated at the end of the CI job, such as an environment variable.

This is for #498

src/constants.ts

@@ -2,7 +2,8 @@ export enum Inputs {
     Key = "key",
     Path = "path",
     RestoreKeys = "restore-keys",
-    UploadChunkSize = "upload-chunk-size"
+    UploadChunkSize = "upload-chunk-size",
+    SkipSave = "skip-save"
 }
 
 export enum Outputs {

src/save.ts

@@ -29,6 +29,13 @@ async function run(): Promise<void> {
             return;
         }
 
+        const skipSave = ["true", "yes"].includes(
+            core.getInput(Inputs.SkipSave).toLowerCase()
+        );
+        if (skipSave) {
+            core.info(`Cache saving was disabled by setting skip-save.`);
+            return;
+        }
         if (utils.isExactKeyMatch(primaryKey, state)) {
             core.info(
                 `Cache hit occurred on the primary key ${primaryKey}, not saving cache.`

src/utils/testUtils.ts

@@ -13,11 +13,13 @@ interface CacheInput {
     path: string;
     key: string;
     restoreKeys?: string[];
+    skipSave?: string;
 }
 
 export function setInputs(input: CacheInput): void {
     setInput(Inputs.Path, input.path);
     setInput(Inputs.Key, input.key);
+    input.skipSave && setInput(Inputs.SkipSave, input.skipSave);
     input.restoreKeys &&
         setInput(Inputs.RestoreKeys, input.restoreKeys.join("\n"));
 }
@@ -25,6 +27,7 @@ export function setInputs(input: CacheInput): void {
 export function clearInputs(): void {
     delete process.env[getInputName(Inputs.Path)];
     delete process.env[getInputName(Inputs.Key)];
+    delete process.env[getInputName(Inputs.SkipSave)];
     delete process.env[getInputName(Inputs.RestoreKeys)];
     delete process.env[getInputName(Inputs.UploadChunkSize)];
 }