Pin go version for windows to go1.15

Tests are failing with go 1.16 at the moment, and by default we test
with latest go version.

https://github.com/prometheus/prometheus/issues/8403

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

.circleci/config.yml

@@ -62,7 +62,7 @@ jobs:
     - run:
         # Temporary workaround until circleci updates go.
         command: |
-          choco upgrade -y golang
+          choco upgrade -y golang --version=1.15.8
     - run:
         command:
           refreshenv