cirrus: Allow the testing user to write to $GOPATH

Cirrus CI caches the module directory inside $GOPATH so it can be shared
between test runs, to speed them up.

However, it is currently created as root, which causes new entries to
fail since they run under the "testing" user.

This patch fixes that problem by making the "testing" user own the
entire $GOPATH, which includes the module directory.

.cirrus.yml

@@ -17,6 +17,8 @@ linux_task:
     folder: $GOPATH/pkg/mod
   get_script: |
     useradd --create-home --user-group testing
+    chown -R testing:testing $GOPATH/
+    su testing -c "go mod download"
     su testing -c "go get ./..."
   build_script: |
     su testing -c "go build ./..."