CI: Check go.sum is up-to-date

[felixge]

go 1.16 made a change causing most go tools to act as if -mod=readonly
was set.

VS Code runs go list by default, so if this file is not up-to-date
contributors using go 1.16 will be greeted with editor errors right
away.

This problem exists for many other go workflows as well, so this patch
adds a CI check to make sure our go.sum stays up-to-date.

[felixge]

@gbbr @knusbaum this is in the same spirit as #917 but hopefully less controversial as a first step towards creating a better Go 1.16 contributor experience.

[felixge]

Here is another workflow error I experienced just now that would be fixed by merging this:

# checked out my delta profile branch
$ go test ./profiler
options.go:26:2: missing go.sum entry for module providing package github.com/DataDog/datadog-go/statsd (imported by gopkg.in/DataDog/dd-trace-go.v1/profiler); to add:
        go get gopkg.in/DataDog/dd-trace-go.v1/profiler
../internal/globalconfig/globalconfig.go:14:2: no required module provides package github.com/google/uuid; to add it:
        go get github.com/google/uuid
$ go get gopkg.in/DataDog/dd-trace-go.v1/profiler
$ go test ./profiler
# gopkg.in/DataDog/dd-trace-go.v1/profiler
metrics_test.go:15:2: missing go.sum entry for module providing package github.com/stretchr/testify/assert (imported by gopkg.in/DataDog/dd-trace-go.v1/profiler); to add:
        go get -t gopkg.in/DataDog/dd-trace-go.v1/profiler
FAIL    gopkg.in/DataDog/dd-trace-go.v1/profiler [setup failed]
$ go get -t gopkg.in/DataDog/dd-trace-go.v1/profiler
$ go test ./profiler
PASS
ok      gopkg.in/DataDog/dd-trace-go.v1/profiler        0.923s

[gbbr]

This is fine by me, but maybe we can squeeze it into one of the other checks?

[knusbaum]

+1 I'm good with this.

Eventually we may want to refactor these CI-related programs into the .circleci directory rather than leave them in the root, but there's only 3 right now so it's not a problem.

[felixge]

@knusbaum @gbbr thanks for review & merging this :)