chore(scripts/update-checksum): make sure $repo always has a value

[TomJo2000]

I may need to split this out into multiple commits.

[TomJo2000]

This PR is temporarily paused pending a request from @Biswa96 to make update-checksum agnostic to the $PWD where it is invoked from. e.g.

# Example 1
cd packages/tmux
$TERMUX_SCRIPT_DIR/scripts/bin/update-checksum
# Updates checksum for tmux

# Example 2
cd packages/vim
$TERMUX_SCRIPT_DIR/scripts/bin/update-checksum . ../../x11-packages/vim-gtk
# Updates checksum for vim and vim-gtk

If that does not cover the usecase(s) you had in mind, please let me know.