docker-run-checks: export INSTALL_ONLY

[trws]

problem: the install-only checks were accidentally running checks instead of only installing

solution: export the INSTALL_ONLY variable so it propagates appropriately

This is a bit of a facepalm mistake. It only actually causes a failure on the arm runners because they run out of time.

[codecov[bot]]

Codecov Report

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

Project coverage is 75.4%. Comparing base (ee1cc87) to head (86f4950). Report is 82 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1279 +/- ## ====================================== Coverage 75.4% 75.4% ====================================== Files 111 111 Lines 15359 15359 ====================================== Hits 11588 11588 Misses 3771 3771 ```

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