Reenable windows upload and fix archive paths

When windows builds fail, we need the local logs for diagnosing the
issue. This commit re-enables archiving the log files from windows
builds for upload. It also fixes that archive to use the correct tar
entry path separator on windows.

relates #115449

Author: rjernst

build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/ElasticsearchBuildCompletePlugin.java

@@ -66,7 +66,7 @@ public void apply(Project target) {
             ? System.getenv("BUILD_NUMBER")
             : System.getenv("BUILDKITE_BUILD_NUMBER");
         String performanceTest = System.getenv("BUILD_PERFORMANCE_TEST");
-        if (buildNumber != null && performanceTest == null && GradleUtils.isIncludedBuild(target) == false && OS.current() != OS.WINDOWS) {
+        if (buildNumber != null && performanceTest == null && GradleUtils.isIncludedBuild(target) == false) {
             File targetFile = calculateTargetFile(target, buildNumber);
             File projectDir = target.getProjectDir();
             File gradleWorkersDir = new File(target.getGradle().getGradleUserHomeDir(), "workers/");
@@ -276,7 +276,12 @@ private static void createBuildArchiveTar(List<File> files, File projectDir, Fil
 
         @NotNull
         private static String calculateArchivePath(Path path, Path projectPath) {
-            return path.startsWith(projectPath) ? projectPath.relativize(path).toString() : path.getFileName().toString();
+            String archivePath = path.startsWith(projectPath) ? projectPath.relativize(path).toString() : path.getFileName().toString();
+            if (OS.current() == OS.WINDOWS) {
+                // tar always uses forward slashes
+                archivePath = archivePath.replace("\\", "/");
+            }
+            return archivePath;
         }
     }
 }