feat: add 'save' input for restore-only mode

[beagleknight]

Summary

• Adds a new save input (default: "true") to action.yml that allows consumers to disable the post-job cache save step
• When save: "false", the save step returns immediately — making the action restore-only

Motivation

In PR workflows at Factorial, cache is already populated by main-branch and scheduled workflows. The save step on PR jobs adds ~4.5 minutes of overhead (tar creation + S3 upload) to jobs whose actual work takes ~20 seconds. This input lets PR workflows skip the save entirely.

Changes

• action.yml: New save input with description, default "true"
• src/save.ts: Early return when save === "false", with info log
• dist/: Rebuilt with yarn build

Usage

- uses: factorialco/actions-cache@v1.7.0
  with:
    key: my-cache-key-${{ hashFiles('...') }}
    restore-keys: my-cache-key-
    path: ./node_modules
    save: 'false'  # restore-only, skip save
    # ... other inputs

After merge

Please tag as v1.7.0 so we can reference it from Factorial's GAT templates.