Clean up tmp files for interrupted `docker build`

This fix tries to fix the issue raised in 29486 where interrupted
`docker build` leaves some tmp files in `/var/lib/docker/tmp`.
With tmp file name prefixed with `/var/lib/docker/tmp/docker-builderXXXXXX`.

The reason for the issue is that in `MakeTarSumContext()`:
```
	if err := chrootarchive.Untar(sum, root, nil); err != nil {
		return nil, err
	}
```
the `err` is shadowed and caused the clean up function in `defer func()`
not being called.

This fix fixes the issue.

This fix is tested manually, as was specified in 29486:
```
rm -rf /var/lib/docker/tmp

mkdir repro && cd repro
fallocate -l 300M bigfile

cat > Dockerfile <<EOF
FROM scratch
COPY ./bigfile /
EOF

docker build .
{Cancel}

ls -la /var/lib/docker/tmp
```

This fix fixes 29486.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

builder/tarsum.go

@@ -104,7 +104,8 @@ func MakeTarSumContext(tarStream io.Reader) (ModifiableContext, error) {
 		return nil, err
 	}
 
-	if err := chrootarchive.Untar(sum, root, nil); err != nil {
+	err = chrootarchive.Untar(sum, root, nil)
+	if err != nil {
 		return nil, err
 	}