feat(gha): cache ko images

This prevents us from seeing outages where the image cannot be
downloaded

actions/setup/action.yml

@@ -46,7 +46,11 @@ runs:
       with:
         go-version: ${{ inputs.go-version }}
         check-latest: ${{ inputs.check-latest }}
-        cache: false  # cache is handled by separate actions/cache step, see https://github.com/actions/setup-go/issues/358
+        cache: false # cache is handled by separate actions/cache step, see https://github.com/actions/setup-go/issues/358
+
+    - name: Set KOCACHE environment variable
+      shell: bash
+      run: echo "KOCACHE=/home/runner/.ko/cache" >> $GITHUB_ENV
 
     # NOTE: cache key and restore key is not the same;
     # We want to always re-use the cache created by master in all our PRs. We also want any PR which modifies go.sum to create a new cache for itself.
@@ -63,6 +67,7 @@ runs:
           /home/runner/.cache/go-build
           /home/runner/go/pkg/mod
           /home/runner/go/bin
+          /home/runner/.ko/cache
         key: ${{ runner.os }}-${{ github.ref_name }}-${{ github.workflow }}-${{ github.job }}-${{ inputs.cacheKey }}-${{ inputs.go-version }}-${{ hashFiles('**/go.sum') }}
         restore-keys: |
           ${{ runner.os }}-${{ github.base_ref }}-${{ github.workflow }}-${{ github.job }}-${{ inputs.cacheKey }}-${{ inputs.go-version }}-${{ hashFiles('**/go.sum') }}