AppVeyor: set custom install prefix

CMake's default has spaces and in 32-bit mode parentheses, which result
in syntax errors in curl-config.

Closes https://github.com/curl/curl/pull/3100

appveyor.yml

@@ -46,7 +46,7 @@ environment:
         SHARED: OFF
 
 build_script:
-    - cmake . -G"%PRJ_GEN%" -DCMAKE_USE_OPENSSL=%OPENSSL% -DBUILD_SHARED_LIBS=%SHARED% -DBUILD_TESTING=%TESTING% -DCURL_WERROR=ON -DCMAKE_RUNTIME_OUTPUT_DIRECTORY_RELEASE="" -DCMAKE_RUNTIME_OUTPUT_DIRECTORY_DEBUG=""
+    - cmake . -G"%PRJ_GEN%" -DCMAKE_USE_OPENSSL=%OPENSSL% -DBUILD_SHARED_LIBS=%SHARED% -DBUILD_TESTING=%TESTING% -DCURL_WERROR=ON -DCMAKE_RUNTIME_OUTPUT_DIRECTORY_RELEASE="" -DCMAKE_RUNTIME_OUTPUT_DIRECTORY_DEBUG="" -DCMAKE_INSTALL_PREFIX="C:/CURL"
     - cmake --build . --config %PRJ_CFG% --clean-first
 
 test_script: