gha: add hosts file workaround for Windows localhost resolution

GitHub Actions Windows Server 2022/2025 runner images ship with the
localhost entries commented out in C:\Windows\System32\drivers\etc\hosts
and rely on the DNS Client service's built-in rule. That path is subject
to Microsoft KB4057932 (getaddrinfo fails with WSAHOST_NOT_FOUND after
an AF_INET6 negative cache), which makes Addrinfo.getaddrinfo("localhost",
53, AF_INET, ...) in mrbgems/mruby-socket/test/addrinfo.rb flaky on CI.

Prepend explicit 127.0.0.1/::1 localhost entries to the hosts file on
all three Windows CI jobs (two mingw-gcc matrix entries and Windows-VC)
before running the build, so the resolver avoids the DNS negative cache
path and returns the expected IPv4 address.

Co-authored-by: Claude <noreply@anthropic.com>

.github/workflows/build.yml

@@ -38,6 +38,12 @@ jobs:
         run: ruby -v
       - name: Compiler version
         run: ${{ env.CC }} --version
+      - name: Ensure localhost resolves on Windows
+        if: runner.os == 'Windows'
+        shell: pwsh
+        run: |
+          Add-Content -Path $env:WINDIR\System32\drivers\etc\hosts -Value "127.0.0.1 localhost"
+          Add-Content -Path $env:WINDIR\System32\drivers\etc\hosts -Value "::1 localhost"
       - name: Build and test
         run: rake -m test:run:serial
 
@@ -79,6 +85,11 @@ jobs:
           persist-credentials: false
       - name: Ruby version
         run: ruby -v
+      - name: Ensure localhost resolves
+        shell: pwsh
+        run: |
+          Add-Content -Path $env:WINDIR\System32\drivers\etc\hosts -Value "127.0.0.1 localhost"
+          Add-Content -Path $env:WINDIR\System32\drivers\etc\hosts -Value "::1 localhost"
       - name: Build and test
         shell: cmd
         run: |