Feature request: option to update cache

Problem Description
Currently, the cache action either restores an existing cache on cache-hit, or generates a missing cache on cache-miss. It does not update the cache on cache-hit.

This works well for caching static dependencies, but not for caching build artefacts.

Proposed Solution
Add an option allowing the user to enable cache updates.
This should be false by default to retain backwards-compatibility.

uses: actions/cache@v2
with:
  path: ccache
  key: ${{ matrix.CONFIG }}-${{ matrix.CXX }}-${{ matrix.TYPE }}
  update: true   # <~~ explicitly request an update

Motivation
Some programming languages benefit greatly from build caching. C++ in conjunction with ccache is the prime example. Using caching commonly decreases compilation times by at least 70%. Medium-sized projects easily take 20 minutes to compile.
ccache also manages the cache size itself and automatically removes obsolete entries, thus the cache won't explode with continuous updates.

It also saves time and money for both user and provider. The environment will be happy too.