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

@@ -97,15 +97,21 @@ bin\nvim --version ; exitIfFailed
 
 # Functional tests
 # The $LastExitCode from MSBuild can't be trusted
-$failed = $false
-# Temporarily turn off tracing to reduce log file output
-Set-PSDebug -Off
-cmake --build . --config $cmakeBuildType --target functionaltest -- $cmakeGeneratorArgs |
-  foreach { $failed = $failed -or
-    $_ -match 'Running functional tests failed with error'; $_ }
-Set-PSDebug -Trace 1
-if ($failed) {
-  exit $LastExitCode
+if ($compiler -eq 'MSVC') {
+  $failed = $false
+  # Temporarily turn off tracing to reduce log file output
+  Set-PSDebug -Off
+  cmake --build . --config $cmakeBuildType --target functionaltest -- $cmakeGeneratorArgs |
+    foreach { $failed = $failed -or
+      $_ -match 'Running functional tests failed with error'; $_ }
+  Set-PSDebug -Trace 1
+  if ($failed) {
+    exit $LastExitCode
+  }
+}
+else {
+  cmake --build . --config $cmakeBuildType --target functionaltest -- $cmakeGeneratorArgs
+  exitIfFailed
 }