Disable pc-windows-gnu CI for now

Currently failing in GitHub Actions with:

    Compiling demo v0.0.0 (D:\a\cxx\cxx\demo)
     Finished dev [unoptimized + debuginfo] target(s) in 1m 57s
      Running `target\debug\demo.exe`
    error: process didn't exit successfully: `target\debug\demo.exe` (exit code: 0xc0000139, STATUS_ENTRYPOINT_NOT_FOUND)
    Error: Process completed with exit code 1.

.github/workflows/ci.yml

@@ -21,9 +21,6 @@ jobs:
           - name: macOS
             rust: nightly
             os: macos
-          - name: Windows (gnu)
-            rust: nightly-x86_64-pc-windows-gnu
-            os: windows
           - name: Windows (msvc)
             rust: nightly-x86_64-pc-windows-msvc
             os: windows