CLI cleanup routines run even if that step isn't reached

[sobjornstad]

E.g.:

$ tzk build public
tzk: Starting build of product 'public'.
tzk: Found 11 build steps.
tzk: Step 1/11: Require a specific Git branch to be checked out
tzk: Step 2/11: Require the working tree of the Git repository to be clean
tzk: ERROR: There are uncommitted changes. Please commit or stash them before publishing (try 'tzk commit').
tzk: Build of product 'public' failed on step 2, backed by builder 'require_clean_working_tree'. 
tzk: Add '--skip-builder require_clean_working_tree' if you'd like to skip this step.
tzk: Running cleanup routine registered by step 3...