[go] Pin tests to use Go 1.20

Go 1.21 was just released but seems to have some issues around
environment variable handling that cause warnings and failures related
to the GOPROXY and GOPATH env vars even when those vars are unset. I
spend a while investigating but couldn't figure out a fix, so I'm
punting on it in case it turns out to be a bug that gets fixed upstream.

Change-Id: Id0f6e6f6e9dea28568deee4712738c946b9dc418
Reviewed-on: https://fuchsia-review.googlesource.com/c/shac-project/shac/+/897162
Fuchsia-Auto-Submit: Oliver Newman <olivernewman@google.com>
Reviewed-by: Marc-Antoine Ruel <maruel@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
diff --git a/scripts/tests.sh b/scripts/tests.sh
index 7241273..894b8d9 100755
--- a/scripts/tests.sh
+++ b/scripts/tests.sh
@@ -33,7 +33,7 @@
   export GOROOT="$CIPD_ROOT/go"
   echo "- Installing Go from CIPD..."
   cipd init -force "$GOROOT"
-  cipd install -log-level error -root "$GOROOT" 'infra/3pp/tools/go/${platform}'
+  cipd install -log-level error -root "$GOROOT" 'infra/3pp/tools/go/${platform}' version:2@1.20.7
   export PATH="$GOROOT/bin:$PATH"
   echo ""
 fi