Refactored 'ARDUINO_SDK_PATH' env variable to 'environment' section in 'AppVeyor'.

Also refactored some env variables defined during the 'install' phase to be standard variables as they're not required to be environmental.
At last removed cache dependency from SDK path to version, at least for now.

Author: MrPointer

appveyor.yml

@@ -5,18 +5,18 @@ environment:
   matrix:
   - ARDUINO_SDK_VERSION: 1.8.2
   - ARDUINO_SDK_VERSION: 1.8.6
+  ARDUINO_SDK_PATH: $pwd\arduino-sdk\arduino-$env:ARDUINO_SDK_VERSION
 cache:
-- '%ARDUINO_SDK_PATH% -> %ARDUINO_SDK_VERSION%'
+- '%ARDUINO_SDK_PATH%'
 install:
 - ps: cinst unzip
-- ps: $env:ARDUINO_SDK_PATH = "$pwd\arduino-sdk\arduino-$env:ARDUINO_SDK_VERSION"
-- ps: $env:ARDUINO_SDK_FILE = "arduino-$env:ARDUINO_SDK_VERSION-windows.zip"
+- ps: $sdk_file = "arduino-$env:ARDUINO_SDK_VERSION-windows.zip"
 - ps: |
     if ( -Not (Test-Path -Path $env:ARDUINO_SDK_PATH) )
     {
-      $env:ARDUINO_SDK_URI = "https://downloads.arduino.cc/$env:ARDUINO_SDK_FILE"
-      wget "$env:ARDUINO_SDK_URI" -O "$env:ARDUINO_SDK_FILE"
-      unzip "$env:ARDUINO_SDK_FILE" -d "arduino-sdk"
+      $arduino_sdk_uri = "https://downloads.arduino.cc/$sdk_file"
+      wget "$arduino_sdk_uri" -O "$sdk_file"
+      unzip "$sdk_file" -d "arduino-sdk"
     }
 - ps: $env:ARDUINO_SDK_PATH = ($env:ARDUINO_SDK_PATH -replace "\\","/")
 before_build: