[`flake8-errmsg`] Avoid shadowing existing `msg` in fix for `EM101` (#24363)
Closes #24335
As suggested, we use the new `fresh_binding` helper from #24316
Note that the issue with this fix was already brought up in the
discussion in #9052 (see also #9059), where it was decided that it was
okay because the fix was already marked as unsafe.