Improve post-build event to only call vsvars32.bat once
Improve post-build event to only call vsvars32.bat once

This should fix a problem on one of the build agents which
had both %VS140COMNTOOLS% and %VS120COMNTOOLS%
but for which %VS120COMNTOOLS%vsvars32.bat failed

StephenMcConnel/geckofxHtmlToPdf,StephenMcConnel/geckofxHtmlToPdf,StephenMcConnel/geckofxHtmlToPdf