Fix delivery local failures by adding 'chef exec' prefix to all commands in project.toml

[afiune]

Description

We have encountered a problem when users install ChefDK for the first time and they do not have it on its PATH env variable, they end-up with the following behavior:

$ chef generate cookbook test
$ cd test
$ delivery local unit
Chef Delivery
Running Unit Phase
thread 'main' panicked at 'Unexpected error: Failed to execute process: No such file or directory (os error 2)', src/delivery/command/local.rs:53

The main problem is that the project.toml doesn't have the prefix chef exec to run the unit phase in the context of ChefDK.

This change is adding the prefix to the default generated project.toml so if the user doesn't have ChefDK configured on its PATH, it wont matter and you will be able to run all phases.

$ delivery local unit
Chef Delivery
Running Unit Phase
.

Finished in 2.59 seconds (files took 1.51 seconds to load)
1 example, 0 failures

Issues Resolved

BUG-345

Check List

• New functionality includes tests
• All tests pass
• RELEASE_NOTES.md has been updated if required (not required for bugfixes, required for API changes)
• All commits have been signed-off for the Developer Certificate of Origin. See https://github.com/chef/chef/blob/master/CONTRIBUTING.md#developer-certification-of-origin-dco

[tpetchel]

Looks good 👍 I verified this change resolves the issue.