Ensure enforce.sh doesn't use unset variables

[Flamefire]

Test this by using set -u in build scripts

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e415c67) to head (79aa619).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #272   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           22        22           
  Branches        12        12           
=========================================
  Hits            22        22           

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[Flamefire]

@sdarwin All looks good now on CI. Can you do a last check and merge this please? Then you should be able to rebase your lcov branch on top and enable set -u which would be a good test for this too

[sdarwin]

• Rebased into lcov branch.
• In terms of merging to master: if you are very confident about set -u for all boost repos using these CI files, then in that case I approve the PR, you may merge.