[LINTER] Enforce shellcheck >= 0.7.0
The recent changes from D6701 require the --source-path option which
is only available from 0.7.0. Since shellcheck can be download as a
standalone binary this is still easy to install.
arc lint --everything
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D6720