feat: custom clone url by OMAKUB_URL environment

[nishidayuya]

This pull-request makes us changeable clone url. a little more feel casual to use customized Omakub.

Test

no OMAKUB_URL case:

I ran following and saw cloning official repository:

$ wget -qO- https://raw.githubusercontent.com/nishidayuya/omakub/refs/heads/feat_custom_clone_url/boot.sh |
    bash -x

set OMAKUB_URL case:

I ran following and saw cloning my forked repository:

$ wget -qO- https://raw.githubusercontent.com/nishidayuya/omakub/refs/heads/feat_custom_clone_url/boot.sh |
    OMAKUB_URL=https://github.com/nishidayuya/omakub.git \
    OMAKUB_REF=feat_batch_installation \
    bash -x

[dhh]

Would wouldn't you just run your own install script from the fork directly then?