ci/win: don't use lastexitcode hack on mingw build

The heck is for MSVC builds to workaround msbuild error detection for cmake.

ci/build.ps1

@@ -97,6 +97,7 @@ bin\nvim --version ; exitIfFailed
 
 # Functional tests
 # The $LastExitCode from MSBuild can't be trusted
+if ($compiler -eq 'MSVC') {
   $failed = $false
   # Temporarily turn off tracing to reduce log file output
   Set-PSDebug -Off
@@ -107,6 +108,11 @@ Set-PSDebug -Trace 1
   if ($failed) {
     exit $LastExitCode
   }
+}
+else {
+  cmake --build . --config $cmakeBuildType --target functionaltest -- $cmakeGeneratorArgs
+  exitIfFailed
+}
 
 
 if ($uploadToCodecov) {