ci: make the advisory license check work on the egress-free runner

The hardened runner's job network has no internet access, so `apk add` and
downloading addlicense from GitHub cannot work. git, curl, jq and addlicense
are baked into the runner's job image (infra/gitea-runner/ci-tools.Dockerfile).

.gitea/workflows/license.yml

@@ -33,16 +33,10 @@ jobs:
     name: addlicense
     runs-on: alpine-latest
     steps:
-      - name: Prerequisites
+      # git, curl, jq and addlicense are baked into the runner's job image
-        run: apk add --no-cache git curl ca-certificates jq
+      # (infra/gitea-runner/ci-tools.Dockerfile in the root repo). The job
-
+      # network has no egress, so `apk add` and downloading addlicense from
-      - name: Install addlicense
+      # the internet cannot work here.
-        run: |
-          set -e
-          curl -fsSL -o /tmp/addlicense.tgz \
-            https://github.com/google/addlicense/releases/download/v1.2.0/addlicense_v1.2.0_Linux_x86_64.tar.gz
-          tar -xzf /tmp/addlicense.tgz -C /tmp
-          install -m 0755 /tmp/addlicense /usr/local/bin/addlicense
 
       - name: Check added files for an AGPL header
         env: