automation: increase root volume size on Linux

It is close to full in the AMI. I actually ran out of space running
tests without increasing the volume size!

Differential Revision: https://phab.mercurial-scm.org/D6716

contrib/automation/hgautomation/aws.py

@@ -970,7 +970,7 b' def temporary_linux_dev_instances(c: AWS'
             'DeviceName': image.block_device_mappings[0]['DeviceName'],
             'Ebs': {
                 'DeleteOnTermination': True,
-                'VolumeSize': 8,
+                'VolumeSize': 12,
                 'VolumeType': 'gp2',
             },
         }