Command line length limit in GenerateProtoTask causes overwriting of proto descriptors

#### **Description**
When compiling a large number of `.proto` files, the generated command exceeds the Default CMD character limit.

```groovy
// Most OSs impose some kind of command length limit.
// Rather than account for all cases, pick a reasonable default of 64K.
static final int DEFAULT_CMD_LENGTH_LIMIT = 65536
```

To circumvent this, the `generateCmds` function splits the command into multiple smaller commands. However, each of these commands writes to the same output descriptor file via `--descriptor_set_out`, causing the last command to overwrite the previous ones.

#### **Affected Code**
[`GenerateProtoTask.groovy` (Lines 187-211)](https://github.com/google/protobuf-gradle-plugin/blob/fb71f679f127aa712a2c5618716110de14d2c2f6/src/main/groovy/com/google/protobuf/gradle/GenerateProtoTask.groovy#L187-L211)

```groovy
static List> generateCmds(List baseCmd, List protoFiles, int cmdLengthLimit) {
List> cmds = []
if (!protoFiles.isEmpty()) {
int baseCmdLength = baseCmd.sum { it.length() + CMD_ARGUMENT_EXTRA_LENGTH } as int
List currentArgs = []
int currentArgsLength = 0
for (File proto: protoFiles) {
String protoFileName = proto
int currentFileLength = protoFileName.length() + CMD_ARGUMENT_EXTRA_LENGTH
if (baseCmdLength + currentArgsLength + currentFileLength > cmdLengthLimit) {
cmds.add(baseCmd + currentArgs) // Adds a command before overflow
currentArgs.clear()
currentArgsLength = 0
}
currentArgs.add(protoFileName)
currentArgsLength += currentFileLength
}
cmds.add(baseCmd + currentArgs)
}
return cmds
}
```

#### **Expected Behavior**
Each split command should write to a unique descriptor file and then merge them, preventing data loss.

#### **Actual Behavior**
Each generated command uses the same `--descriptor_set_out` parameter, leading to overwriting instead of appending.

#### **Steps to Reproduce**
1. Compile a large number of `.proto` files that exceed the CMD character limit.
2. Observe that multiple commands are executed.
3. Check the final descriptor file – it contains only the last batch of `.proto` files.

#### **Proposed Fix**
- Modify the `--descriptor_set_out` path for each split command.
- Merge the descriptor files after all commands are executed.

Would appreciate feedback or any alternative suggestions! 🚀