Fix MSVC recipe for ecpg regression tests.

Fix MSVC recipe for ecpg regression tests.

While back-patching commit 6f0cef935, I forgot that the MSVC
build scripts would also need adjustment in the back branches.
This is a blind attempt at a fix, but it's basically copying
nearby code so I think it will work.

Per buildfarm (via Andrew Dunstan)

Discussion: https://postgr.es/m/4cc4dc47-ca2b-4129-8784-db69b5f82777@dunslane.net