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

This reverts commit bc43d2559f.

stderr redirection should make the workaround work for mingw builds.

ci/build.ps1

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