feat: increase rootfs tools size from 100MB to 256MB

[edubart]

This fixes #32

Currently the rootfs is too small for playing with VirtIO networking, you can't use apt-get update && apt-get install -y python for example because there is not enough disk space.

I believe we should make easier to let people experiment trying out packages with less friction using the rootfs we provide as example, the increase from 100MB to 256MB is enough to allow people installing and experimenting python and other packages.

For example, after this PR, you can execute the following without errors:

$ cartesi-machine --network --append-init=USER=root -it "apt-get update && apt-get install -y python && python"
...
Python 3.10.12 (main, Nov 20 2023, 15:14:05) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 

[vfusco]

Another solution was proposed truncating the file and using resize2fs inside of the machine when this is needed.