Fix various linter issues

Summary:
Running arc lint everything returned me 2 warnings, 1 from shellcheck
and one from clang-format.

Test Plan:
Should return no error nor warning:

arc lint --everything

Run some CI build.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4362