Fix race condition when saving caches

In the previous code there was a potential race condition when two or
more workflows checked the cache at the same time:

1. Workflow 1: cache miss; Workflow 2: cache miss
2. Workflow 1: install; Workflow 2: install
3. Workflow 1: save cache; Workflow 2: save cache <<< RACE CONDITION

This resulted in the step failing with the following error:

Error: reserveCache failed: Cache already exists. Scope: refs/heads/master,
Key: <key>, Version: <...>

It's a bug in the @actions/cache package and it's discussed in the
following issue: actions/toolkit#537

The correct behaviour would be for Workflow 2 to simply drop its attempt
to save the cache entry when this situation occurs.

For now, this code uses a workaround to fix this issue which works by
detecting and catchin the above error based on its error message.

index.js

@@ -20,8 +20,8 @@ async function main() {
       version = execSync('npm view ajv-cli version').toString().trim();
       core.info(`Latest version of ajv-cli is ${version}`);
     } else if (!execSync('npm view ajv-cli versions').includes(version)) {
-    fail(`${version} is not a valid version of ajv-cli (list available versions with 'npm view ajv-cli versions'`);
-  }
+      fail(`${version} is not a valid version of ajv-cli`);
+    }
 
     key = `ajv-cli-${version}-${process.env.RUNNER_OS}`;
     installDir = `${process.env.RUNNER_TEMP}/${key}`;
@@ -42,7 +42,16 @@ async function main() {
       core.endGroup();
       // Save installation directory to cache
       core.startGroup(`Caching installation directory with key ${key}`);
-      await cache.saveCache([installDir], key);
+      try {
+        await cache.saveCache([installDir], key);
+      // Workaround for race condition: https://github.com/actions/toolkit/issues/537
+      } catch (err) {
+        if (err.message.includes("Cache already exists")) {
+          core.info(`Cache entry ${key} has already been created by another worfklow`);
+        } else {
+          throw err;
+        }
+      }
       core.endGroup();
     // If cache has been found and restored
     } else {