fix appveyor config

AppVeyor sets the latest go stack in C:\go and that is GOROOT.
Cloning this repo under C:\go\src\.. directory confused go build
(as discussed in golang/go#34657). The fix in go is not yet
released. Thus this commit changes GOPATH to c:\gopath and clones
the repo under the directory.

Also, this commit removes the go get commands intended to pull in
dependencies. In modules mode, `go build` pulls in the required
dependencies.

appveyor.yml

@@ -1,13 +1,13 @@
-clone_folder: c:\go\src\github.com\google\pprof
+clone_folder: c:\gopath\src\github.com\google\pprof
+
+environment:
+  GOPATH: c:\gopath
 
 install:
  - cinst graphviz
 
-before_build:
- - go get github.com/ianlancetaylor/demangle
- - go get github.com/chzyer/readline
-
 build_script:
+ - go env
  - go build github.com/google/pprof
 
 test_script: