ci(windows): resolve host gateway at runtime instead of hardcoding 172.17.0.1

Forgejo Runner spawns each job on a per-job custom Docker network whose
gateway is not the default bridge (172.17.0.1). Detecting the default
route inside the container picks up whatever bridge gateway is in use.

.forgejo/workflows/ci.yml

@@ -5,7 +5,9 @@ on:
   workflow_dispatch:
 
 env:
-  WIN_HOST: 172.17.0.1
+  # WIN_HOST is the host machine reachable from inside the job container.
+  # Resolved at runtime to the container's default gateway, since Forgejo Runner
+  # creates a per-job Docker network whose gateway IP varies between runs.
   WIN_PORT: 2210
   WIN_USER: edr
 
@@ -28,7 +30,13 @@ jobs:
       - name: Install sshpass
         run: |
           apt-get update -qq
-          apt-get install -y -qq sshpass openssh-client
+          apt-get install -y -qq sshpass openssh-client iproute2
+
+      - name: Resolve host gateway
+        run: |
+          GW=$(ip route | awk '/default/ {print $3; exit}')
+          echo "WIN_HOST=$GW" >> "$GITHUB_ENV"
+          echo "Using host gateway: $GW"
 
       - name: Wait for Windows VM SSH
         env: