[LINTER] Silent a shellcheck false positive

Summary:
The advice here is not applicable:

This backslash+linefeed is literal. Break outside single quotes if you
just want to break the line.

Test Plan:

arc lint --everything

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D5201