Set ZEBRA_SKIP_NETWORK_TESTS using Windows syntax (#1782)

Use Powershell syntax to set ZEBRA_SKIP_NETWORK_TESTS on Windows.

Also skip the entire large sync test step on Ubuntu and
Windows, because the tests are skipped anyway due to
ZEBRA_SKIP_NETWORK_TESTS. This saves some 
compilation time.

.github/workflows/ci.yml

@@ -36,11 +36,14 @@ jobs:
       - name: Install LLVM on Windows
         if: matrix.os == 'windows-latest'
         run: choco install llvm -y
-      - name: Skip network tests on Ubuntu and Windows
+      - name: Skip network tests on Ubuntu
         # Ubuntu runners don't have network or DNS configured during test steps
-        # Windows runners have an unreliable network
-        if: matrix.os == 'ubuntu-latest' || matrix.os == 'windows-latest'
+        if: matrix.os == 'ubuntu-latest'
         run: echo "ZEBRA_SKIP_NETWORK_TESTS=1" >> $GITHUB_ENV
+      - name: Skip network tests on Windows
+        # Windows runners have an unreliable network
+        if: matrix.os == 'windows-latest'
+        run: echo "ZEBRA_SKIP_NETWORK_TESTS=1" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
       - name: Show env vars
         run: |
             echo "ZEBRA_SKIP_NETWORK_TESTS=${{ env.ZEBRA_SKIP_NETWORK_TESTS }}"
@@ -51,8 +54,10 @@ jobs:
         with:
           command: test
           args: --verbose --all
-      # Explicitly run any tests that are usually #[ignored], modulo ZEBRA_SKIP_NETWORK_TESTS
+      # Explicitly run any tests that are usually #[ignored]
       - name: Run zebrad large sync tests
+        # Skip the entire step on Ubuntu and Windows, because the test would be skipped anyway due to ZEBRA_SKIP_NETWORK_TESTS
+        if: matrix.os == 'macOS-latest'
         uses: actions-rs/cargo@v1
         with:
           command: test