Makefile: fix RPM build

Makefile: fix RPM build

`make rpm` usually chose `/tmp` as the `$rpmbuild` dir, which breaks the
dracut build, since it needs to execute `./configure`, but `/tmp` is
mounted with `-o noexec`, leading to:

```
/var/tmp/rpm-tmp.SwEhJO: line 46: ./configure: Permission denied
error: Bad exit status from /var/tmp/rpm-tmp.SwEhJO (%build)

RPM build errors:
    Bad exit status from /var/tmp/rpm-tmp.SwEhJO (%build)
```

Switching to `/var/tmp` helps in this case.