chore(toolkit-lib): add caret to aws-cdk/cli-plugin-contract devDep to unblock release (#496)

At the end of the release process build we are restoring versions for
local monorepo packages. This is currently hard-coded to always include
a caret `^`. While not ideal, it doesn't matter in our case. Before we
do a proper fix, unblock the release by just adding the caret.

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Author: mrgrain