Added support for export LC_ALL=C.UTF-8 to the shell linter

Summary:
Similar in spirit to https://github.com/bitcoin/bitcoin/pull/13863/commits/414326952cd6086ebc491413e8cf58a60ad857a4#diff-41e4d53455228facb9cfa198baaa2440

This allows us to explicitly set the codeset to UTF-8.

Test Plan:
arc lint on the following cases:

• script with export LC_ALL=C
• script with no LC_ALL set. Fails as expected.
• script with export LC_ALL=C.UTF-8
• script with export LC_ALL=C.UTF-9 Fails as expected (sanity check that we don't accidentally match on export LC_ALL=C)

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4183