Kill ripgrep process when file search is cancelled.

Previously we only cancelled the part of the search that happens in our process, not the actual rg process.
Fix #135936

src/vs/workbench/services/search/node/fileSearch.ts

@@ -102,6 +102,7 @@ export class FileWalker {
 
 	cancel(): void {
 		this.isCanceled = true;
+		killCmds.forEach(cmd => cmd());
 	}
 
 	walk(folderQueries: IFolderQuery[], extraFiles: URI[], onResult: (result: IRawFileMatch) => void, onMessage: (message: IProgressMessage) => void, done: (error: Error | null, isLimitHit: boolean) => void): void {