[CLI] Don't create a temp workspace on CI

[jhnstn]

Fixes #174

We won't need a temp directory on a CI assuming we are spinning up a new VM on each run (like what happens on Github Workflows)

Instead we can just use the directory set up by the CI to run the commands.

This adds a check for the CI env before creating a temp directory. Since a temp directory is not created, nothing is deleted after the command runs

Testing

• Create a build to some empty directory e.g. go build ~/some-test-directory/gbm-cli
• cd into that directory and create a new empty directory gb
• From the directory created in the last step run CI=true ../gbm-cli release prepare gb {version} --no-tag
• Notice a message about not creating a temp directory
• Exit before creating the PR
• Note that Gutenberg is cloned to the current directory
• Verify that nothing was deleted at the end of the command run