Use #[marker] on nightly in CI

[joshlf]

We eventually hope to make use of #[marker] traits once they're
stable. This permits us to test to make sure the feature is as we expect
and that our intended usage works.

---

This PR is on branch ptr-overhaul.

• [pointer] Support Box and Arc #1913
• [pointer][transmute] Support generic TransmuteFrom #1910
• [pointer][transmute] Migrate from util #1911
• Use #[marker] on nightly in CI #1925

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.46%. Comparing base (f04c344) to head (f060543).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1925      +/-   ##
==========================================
- Coverage   89.47%   89.46%   -0.02%     
==========================================
  Files          16       16              
  Lines        5846     5838       -8     
==========================================
- Hits         5231     5223       -8     
  Misses        615      615              

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