`trap` can not access the value of a variable defined as local once the
function body is left. So we use the value of the variable at parse time
instead of trying to access the value at runtime of the `trap` command.
To inspect the state of the application it is useful to checkout the
directory content of the temporary working directory. Setting the DEBUG
variable to either 1 or true stops bootstrap.sh to delete that directory,
which it does regardless if the app finished successfully or because of
an error.
Having to clone from github or a remote repository in general during
development is a waste of bandwidth, time and does not work during
development while a change is being tested and not yet commited or
pushed. Now remote urls can be configured through two env variables:
MACOS_SYSTEM_REPO_URL and ZSHLIB_REPO_URL.
