ci(stt): don't fail the build when the runner's gha cache is unreachable

The STT job failed on the home runner with 'no route to host' to the
act_runner cache service while exporting the build cache. Import errors
are warnings, export errors are fatal without ignore-error=true, which
build.yml already sets. Ephemeral runners hit the same thing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitea/workflows/build-stt.yml

@@ -55,5 +55,10 @@ jobs:
             ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ gitea.sha }}
             ghcr.io/shadowdao/triple-c-stt:latest
             ghcr.io/shadowdao/triple-c-stt:${{ gitea.sha }}
+          # The gha cache is served by the runner that picks the job up, and
+          # is unreachable from the job on some of ours (the home box's
+          # firewalled per-job bridge; ephemeral runners). Import failures
+          # only warn, but an export failure fails the build unless told not
+          # to -- the same setting build.yml already carries.
           cache-from: type=gha
-          cache-to: type=gha,mode=max
+          cache-to: type=gha,mode=max,ignore-error=true