Increase worker EBS volume to 12GB

These instances start up 99% full as soon as they are built. The
usage breakdown is as follows:

```
$ df -h /

Filesystem      Size  Used Avail Use% Mounted on
/dev/root       7.6G  7.5G  141M  99% /
```
```
$ sudo du -hs /*

...
3.8G	usr
5.3G	var
```
```
$ sudo du -sh /usr/* | grep M

406M	/usr/bin
215M	/usr/include
 99M	/usr/libexec
670M	/usr/local
 34M	/usr/sbin
332M	/usr/share
285M	/usr/src
```
```
$ sudo du -sh /var/lib/* | grep G

4.3G	/var/lib/docker
```

It is likely that the size of dependencies has grown over time,
particularly Python libraries and Docker executables. This leaves
little room for operating the instances, which fill up sooner and
sooner, necessitating manual termination and restarts.

This increased space will give us more breathing room for the
next few years.

deployment/packer/mmw.json

@@ -62,6 +62,12 @@
             "ssh_username": "ubuntu",
             "ami_name": "mmw-worker-{{timestamp}}-{{user `version`}}",
             "ami_block_device_mappings": [
+                {
+                    "device_name": "/dev/sda1",
+                    "volume_type": "gp2",
+                    "volume_size": 12,
+                    "delete_on_termination": true
+                },
                 {
                     "device_name": "/dev/sdf",
                     "snapshot_id": "snap-0211cbbff8a81266f",