🐛 Atomically load from accessState to avoid data race.

[spencerschrock]

What kind of change does this PR introduce?

bug fix

• PR title follows the guidelines defined in our pull request documentation

What is the current behavior?

There's a data race in the main binary, which can be seen when built with go build -race

==================
WARNING: DATA RACE
Write at 0x00c0001204a8 by goroutine 43:
  sync/atomic.CompareAndSwapInt64()
      /usr/lib/go-1.19/src/runtime/race_amd64.s:316 +0xb
  sync/atomic.CompareAndSwapInt64()
      <autogenerated>:1 +0x24
  github.com/ossf/scorecard/v4/clients/githubrepo/roundtripper.(*githubTransport).RoundTrip()
      ~/go/src/github.com/spencerschrock/scorecard.git/clients/githubrepo/roundtripper/transport.go:44 +0x8e
      
Previous read at 0x00c0001204a8 by goroutine 60:
  github.com/ossf/scorecard/v4/clients/githubrepo/roundtripper/tokens.(*roundRobinAccessor).Next()
      ~/go/src/github.com/spencerschrock/scorecard.git/clients/githubrepo/roundtripper/tokens/round_robin.go:39 +0x1a6

What is the new behavior (if this is a feature change)?**

The load is done atomically to avoid the race.

• Tests for the changes have been added (for bug fixes/features)

Which issue(s) this PR fixes

Unblocks #2625

Special notes for your reviewer

Does this PR introduce a user-facing change?

For user-facing changes, please add a concise, human-readable release note to
the release-note

(In particular, describe what changes users might need to make in their
application as a result of this pull request.)

[codecov]

Codecov Report

Merging #2732 (1020927) into main (0c090b3) will not change coverage.
The diff coverage is 0.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2732   +/-   ##
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  Coverage   49.87%   49.87%           
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  Files         156      156           
  Lines       11572    11572           
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  Hits         5772     5772           
  Misses       5450     5450           
  Partials      350      350