ci: make GHA cache export non-fatal (ignore-error=true)

The frontend/backend image builds + pushes succeed, then the final
'exporting to GitHub Actions Cache' step intermittently fails with
'error writing layer blob: not_found' (a known flaky type=gha cache backend
issue), failing the whole job. Add ignore-error=true to every cache-to so a
cache-write hiccup can't break an otherwise-successful, already-pushed build.

.github/workflows/install-smoke.yml

@@ -61,7 +61,8 @@ jobs:
           load: true
           tags: picpeak-backend:smoke
           cache-from: type=gha,scope=install-smoke
-          cache-to: type=gha,mode=max,scope=install-smoke
+          # ignore-error: a flaky GHA cache write must not fail the build.
+          cache-to: type=gha,mode=max,scope=install-smoke,ignore-error=true
 
       - name: Create Docker network
         run: docker network create picpeak-smoke