access env var from shell

Author: masci

Diff for: appveyor.yml

@@ -18,7 +18,7 @@ stack: go 1.12
 install:
   # install the task executor
   - curl -o task.zip -LO https://github.com/go-task/task/releases/download/v2.6.0/task_windows_amd64.zip
-  - 7z e task.zip -o$(GOPATH)\bin
+  - 7z e task.zip -o%GOPATH%\bin
   # golang dependencies needed at test time
   - go get github.com/golangci/govet
   - go get golang.org/x/lint/golint
@@ -28,7 +28,7 @@ install:
   # protobuf tooling needed at test time
   - go get github.com/golang/protobuf/protoc-gen-go
   - curl -o protoc.zip -LO https://github.com/protocolbuffers/protobuf/releases/download/v3.8.0/protoc-3.8.0-win64.zip
-  - 7z e protoc.zip -o$(PROTOC_PATH)
+  - 7z e protoc.zip -o%PROTOC_PATH%
 
 test_script:
   - task.exe test-integration