[LINTER] Enforce shellcheck >= 0.7.0

Summary:
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.

Test Plan:

arc lint --everything

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6720