system: set TMPDIR before launching QEMU

QEMU uses g_get_tmp_dir() to get the tmp directory.

Fixes #4882

Managers/UTMQemuSystem.m

@@ -51,6 +51,7 @@ static void *start_qemu(void *args) {
         envp[i] = environment[i].UTF8String;
     }
     envp[envc] = NULL;
+    setenv("TMPDIR", NSFileManager.defaultManager.temporaryDirectory.path.UTF8String, 1);
     
     int argc = (int)qemuArgv.count + 1;
     const char *argv[argc];

QEMUHelper/QEMUHelper.m

@@ -121,9 +121,12 @@
     task.arguments = newArgv;
     task.standardOutput = standardOutput;
     task.standardError = standardError;
+    NSMutableDictionary<NSString *, NSString *> *environment = [NSMutableDictionary dictionary];
+    environment[@"TMPDIR"] = NSFileManager.defaultManager.temporaryDirectory.path;
     if (self.environment) {
-        task.environment = self.environment;
+        [environment addEntriesFromDictionary:self.environment];
     }
+    task.environment = environment;
     task.qualityOfService = NSQualityOfServiceUserInitiated;
     task.terminationHandler = ^(NSTask *task) {
         BOOL normalExit = task.terminationReason == NSTaskTerminationReasonExit && task.terminationStatus == 0;