fix matrix report aggregation bug

We can't use ```endRun```, because endRun is not async. endRun isn't executed right after matrix build finishing, but it's executed according its queue. So, with using jclouds, often endRun is executed, when slave is shutdown already
do you mean to make that?:
```
public FilePath getAggregationDir() {
String curBuildNumber = Integer.toString(build.getNumber());
return build.getWorkspace().child(REPORT_DIR_PREFIX + curBuildNumber);
}
```

I thought, you meant:

```
public FilePath getAggregationDir() {
String curBuildNumber = Integer.toString(build.getNumber());
aggregatedAllureFilePath = build.getWorkspace().child(REPORT_DIR_PREFIX + curBuildNumber);
return aggregatedAllureFilePath.child(ReportGenerator.RESULTS_PATH);
}
```