fix: make gapic-generator-java output zip timestamps deterministic

Set a fixed timestamp on all output Jar entries. This follows the approach of [Bazel's Jar creation logic](https://github.com/bazelbuild/bazel/blob/0327100c98a126b9fc83223ba41303fe8b37d331/src/java_tools/buildjar/java/com/google/devtools/build/buildjar/jarhelper/JarHelper.java#L43-L55).

Fixes https://github.com/googleapis/sdk-platform-java/issues/4096

Author: benjaminp

gapic-generator-java/src/main/java/com/google/api/generator/gapic/protowriter/Writer.java

@@ -124,7 +124,7 @@ static void writeReflectConfigFile(
     String jarEntryLocation =
         String.format("src/main/resources/META-INF/native-image/%s/reflect-config.json", pakkage);
     try {
-      jos.putNextEntry(new JarEntry(jarEntryLocation));
+      jos.putNextEntry(jarEntry(jarEntryLocation));
       jos.write(prettyGson.toJson(reflectConfigInfo).getBytes(StandardCharsets.UTF_8));
     } catch (IOException e) {
       throw new GapicWriterException("Could not write reflect-config.json", e);
@@ -141,7 +141,7 @@ private static String writeClazz(
 
     String path = getPath(clazz.packageString(), clazz.classIdentifier().name());
     String className = clazz.classIdentifier().name();
-    JarEntry jarEntry = new JarEntry(String.format("%s/%s.java", path, className));
+    JarEntry jarEntry = jarEntry(String.format("%s/%s.java", path, className));
     try {
       jos.putNextEntry(jarEntry);
       jos.write(code.getBytes(StandardCharsets.UTF_8));
@@ -159,7 +159,7 @@ private static void writeSamples(
       GapicClass gapicClazz, String pakkage, String clazzPath, JarOutputStream jos) {
     for (Sample sample : gapicClazz.samples()) {
       JarEntry jarEntry =
-          new JarEntry(
+          jarEntry(
               String.format(
                   "samples/snippets/generated/%s/%s/%s/%s.java",
                   clazzPath,
@@ -189,7 +189,7 @@ static String writePackageInfo(
     codeWriter.clear();
 
     String packagePath = "src/main/java/" + packageInfo.pakkage().replaceAll("\\.", "/");
-    JarEntry jarEntry = new JarEntry(String.format("%s/package-info.java", packagePath));
+    JarEntry jarEntry = jarEntry(String.format("%s/package-info.java", packagePath));
     try {
       jos.putNextEntry(jarEntry);
       jos.write(code.getBytes(StandardCharsets.UTF_8));
@@ -201,7 +201,7 @@ static String writePackageInfo(
 
   private static void writeMetadataFile(GapicContext context, String path, JarOutputStream jos) {
     if (context.gapicMetadataEnabled()) {
-      JarEntry jarEntry = new JarEntry(String.format("%s/gapic_metadata.json", path));
+      JarEntry jarEntry = jarEntry(String.format("%s/gapic_metadata.json", path));
       try {
         jos.putNextEntry(jarEntry);
         jos.write(
@@ -230,4 +230,10 @@ private static String getPath(String pakkage, String className) {
   private static String getSamplePackage(GapicClass gapicClazz) {
     return gapicClazz.classDefinition().packageString().concat(".samples");
   }
+
+  private static JarEntry jarEntry(String name) {
+    JarEntry entry = new JarEntry(name);
+    entry.setTime(1262304000000L);
+    return entry;
+  }
 }