ci: use -p=8 in GOFLAGS to reduce linker OOMs

The environment Go runs in thinks there are 32 CPUs, but according to
https://circleci.com/docs/2.0/configuration-reference/#resource_class
that's a known issue, and only 2 CPUs are actually available.

We're still overallocating with -p=8, so if we continue to get OOMs we
may need to reduce that further.

.circleci/config.yml

@@ -29,7 +29,7 @@ jobs:
       - image: circleci/golang:1.11
     environment:
       GOCACHE: /tmp/go-cache
-      GOFLAGS: "-mod=readonly"
+      GOFLAGS: "-mod=readonly -p=8" # Go on Circle thinks 32 CPUs are available, but there aren't.
     working_directory: /go/src/github.com/influxdata/platform
     steps:
       - checkout
@@ -81,7 +81,7 @@ jobs:
       - image: circleci/golang:1.11-node-browsers
     environment:
       GOCACHE: /tmp/go-cache
-      GOFLAGS: "-mod=readonly"
+      GOFLAGS: "-mod=readonly -p=8" # Go on Circle thinks 32 CPUs are available, but there aren't.
     working_directory: /go/src/github.com/influxdata/platform
     steps:
       - checkout