ci: skip copyright-lint from GHA

Pre-commit action defaults to run with '--all-files', which is great for
the other action. For copyright-lint however, it is wrong to assume that
all files were changed by a single entity and therefore this can't be
tested easily. It could work if a range of files were passed and the
author email extracted from each commit.

scripts/copyright-lint.sh

@@ -10,6 +10,10 @@
 
 set -eu
 
+if test -n "${GITHUB_ACTION:-}"; then
+  exit 0
+fi
+
 command -v git >/dev/null ||
   { printf '%s\n' "Missing program: git" >&2; exit 1; }
 repo_toplevel="$(git rev-parse --show-toplevel)"