workflows: disable docker-clean so apt cache survives between runs

Debian-based Docker images install /etc/apt/apt.conf.d/docker-clean
which runs 'apt-get clean' after every apt-get install — wipes out
/var/cache/apt/archives so there's nothing for actions/cache to save.

Fix: remove the docker-clean drop-in and add keep-cache config first
thing in each job, before any apt activity. Also bump apt cache key
to v2 since the previous v1 was always empty.

Wait time on the Wine + Inno Setup step should drop from ~1m20s to
~10s on warm runs (debs already in /var/cache/apt/archives, dpkg
just installs them locally).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -25,6 +25,11 @@ jobs:
   test:
     runs-on: go
     steps:
+      - name: Disable apt auto-clean (preserve cache for actions/cache)
+        run: |
+          rm -f /etc/apt/apt.conf.d/docker-clean
+          echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache
+
       - name: Install Node (required by actions/cache)
         run: |
           apt-get update >/dev/null
@@ -61,6 +66,11 @@ jobs:
     runs-on: go
     needs: test
     steps:
+      - name: Disable apt auto-clean (preserve cache for actions/cache)
+        run: |
+          rm -f /etc/apt/apt.conf.d/docker-clean
+          echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache
+
       - name: Install Node (required by actions/cache)
         run: |
           apt-get update >/dev/null