.github: remove github.job from the cache key and update docs

This commit adds a more descriptive docs about the caching process. In
addition, the `github.job` is now removed from the cache key
construction to allow jobs sharing the same cache. In addition, given we
now use `inputs.key-prefix`, the `github.job` is no longer relevant as
the `key-prefix` already creates a unique name for the cache.

We also update the lint and check commits job to use the same cache from
the unit test, given they are running in the same environment.

.github/workflows/main.yml

@@ -126,6 +126,8 @@ jobs:
         uses: ./.github/actions/setup-go
         with:
           go-version: '${{ env.GO_VERSION }}'
+          # Use the same cache from unit test job to save time.
+          key-prefix: unit-test
 
       - name: fetch and rebase on ${{ github.base_ref }}
         uses: ./.github/actions/rebase
@@ -152,6 +154,8 @@ jobs:
         uses: ./.github/actions/setup-go
         with:
           go-version: '${{ env.GO_VERSION }}'
+          # Use the same cache from unit test job to save time.
+          key-prefix: unit-test
 
       - name: lint
         run: GOGC=50 make lint