cmd/objbench: emit warning when testing on url-like file path

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 35.55%. Comparing base (fbe92e3) to head (dab1b65). Report is 6 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4933 +/- ## ========================================== - Coverage 44.35% 35.55% -8.81% ========================================== Files 102 127 +25 Lines 19032 35084 +16052 ========================================== + Hits 8442 12474 +4032 - Misses 9815 21350 +11535 - Partials 775 1260 +485 ```

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