SC2098 not always "technically" correct

With
```
# !/bin/bash
VAR=aaa VAR=${VAR}:bbb bash -c 'echo ${VAR}'
```

Your result will depend on the version of bash you are running:
bash3:
```
:bbb
```
(Sometimes. Try doing
```
echo $VAR; VAR=aaa VAR=${VAR}:bbb bash -c 'echo ${VAR}'
```
to see a different result on OSX)

bash4:
```
aaa:bbb
```

Either way it's great shellcheck warns about this unstable behaviour!
