[ci] setup gopath in /tmp/go

it turns out that "full path" is still a relative path, and therefore
not allowed as a GOPATH.

.gitlab-ci.yml

@@ -81,8 +81,8 @@ osx_installer:
   stage: build
   allow_failure: true
   variables:
-      GOPATH: $CI_PROJECT_DIR/go/
-      APP_PATH: $CI_PROJECT_DIR/go/src/0xacab.org/leap/bitmask-systray
+      GOPATH: /tmp/go/
+      APP_PATH: /tmp/go/src/0xacab.org/leap/bitmask-systray
   script:
     - git clone https://0xacab.org/leap/riseup_vpn
     - cd riseup_vpn