Reduce appveyor churn

appveyor.yml

@@ -9,6 +9,7 @@ environment:
   - TARGET: x86_64-pc-windows-msvc
   - TARGET: i686-pc-windows-msvc
 install:
+  - if NOT defined APPVEYOR_PULL_REQUEST_NUMBER if "%APPVEYOR_REPO_BRANCH%" == "master" appveyor exit
   - if defined MSYS_BITS set PATH=C:\msys64\mingw%MSYS_BITS%\bin;C:\msys64\usr\bin;%PATH%;
   - ps: Start-FileDownload "https://static.rust-lang.org/dist/rust-nightly-${env:TARGET}.exe"
   - rust-nightly-%TARGET%.exe /VERYSILENT /NORESTART /DIR="C:\Program Files (x86)\Rust"