Prior to this patch, Dotbot was relying on running with the base
directory being the current working directory. In practice, it was
relying on the install shim to set up this context. It makes more sense
sense to actually execute `chdir()` within Dotbot itself, rather than
relying on the install shim to do so.
Because 'bin/dotbot' is a hybrid sh/Python script that finds the
appropriate Python binary and then runs, the test code should not invoke
it as an sh script when sh does not source the pyenv initialization
script.
The fix implemented in this patch is to directly run 'bin/dotbot' as a
Python script using 'python', which when called from bash, will run with
the correct Python version (because bash is set up to source the pyenv
initialization script).