[ci] Fix quoting in clang-format CI check
[ci] Fix quoting in clang-format CI check

$changed_files should be quoted when used in `test -z`. This commit
fixes that.

Signed-off-by: Alex Bradbury <ed4b35f7901fdce94913288395fdd0b59e44c43e@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan