cmd/go: go mod tidy and go mod download don't agree on same go.sum file.

[c4milo]

What version of Go are you using (go version)?

go version go1.16 darwin/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output

$ ~/Downloads/go\ 2/bin/go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/caguilar/Library/Caches/go-build"
GOENV="/Users/caguilar/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/caguilar/go/pkg/mod"
GONOPROXY="slack-github.aaakk.us.kg"
GONOSUMDB="slack-github.aaakk.us.kg"
GOOS="darwin"
GOPATH="/Users/caguilar/go"
GOPRIVATE="slack-github.aaakk.us.kg"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/caguilar/Downloads/go 2"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/caguilar/Downloads/go 2/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.16"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/caguilar/src/goslackgo/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/sd/dhdfq5qx471__68g8fl9v_jh0000gp/T/go-build540412494=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

1. go mod tidy
2. go mod download

What did you expect to see?

I was expecting go.sum to not change.

What did you see instead?

go.sum changed. This causes issues in our CI since we don't expect source files to be arbitrarily changed during CI.

[tmthrgd]

This is a duplicate of #43994.