Fix GitHub Actions Windows CI builds' failure to find Boost

[claremacrae]

This build ran fine (both are on the Options branch - there are no changes to our build configuration in between...)

• last good build:
  • https://github.com/approvals/ApprovalTests.cpp/actions/runs/83255621
  • Mon, 20 Apr 2020 21:32:29 GMT
  • Current runner version: '2.169.0'
  • Installed software:
  • https://github.com/actions/virtual-environments/blob/win19/20200319.1/images/win/Windows2019-Readme.md
• First bad build:
  • https://github.com/approvals/ApprovalTests.cpp/actions/runs/84176721
  • Tue, 21 Apr 2020 21:30:19 GMT
  • Current runner version: '2.169.0'
  • Installed software:
  • https://github.com/actions/virtual-environments/blob/win19/20200416.1/images/win/Windows2019-Readme.md

So the change was something to do with the update of the Windows images from 20200319.1 to 20200416.1

[claremacrae]

This will likely have been the cause: Environment variable BOOST_ROOT will be deprecated in 2 month.

Others have found the problem: https://github.com/actions/virtual-environments/issues/749 - which says that there are fixes given in:

• https://github.com/actions/virtual-environments/issues/661
• https://github.com/actions/virtual-environments/issues/687

[claremacrae]

I've fixed this on the Options branch - having squashed together my various attempts at fixing it...

And I've cherry-picked the fix to the master branch.

Closing issue.