SConstruct : Fix slash error on Windows

[ericmehl]

This fixes an error on Windows related to the escaping of backslashes that prevented USD from loading IECoreUSD.dll. It was testing correctly in the past because the tests use a different pluginfo.json than what is installed, which was formatted correctly.

Breaking Changes

None

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Cortex project's prevailing coding style and conventions.

[johnhaddon]

LGTM. Not sure why the Windows CI failed - seems unrelated to this. Also needs a rebase to fix a merge conflict..

[ericmehl]

I rebased it onto 10.3 and retargeted the PR to that also. Technically this is a bug fix and I suppose there may be someone out there using 10.3 on Windows.

Would it make more sense / be easier to just keep it on 10.4?

[johnhaddon]

Would it make more sense / be easier to just keep it on 10.4?

I suspect it's not needed on 10.3, but there's no bother having it there - we merge everything through from 10.3 to 10.4 regularly anyway...