Don't use mapfile to be compatible with Bash 3 on MacOS

This does assume that the path `target/*/build/bandwhich-*/stderr` contains no
spaces, but it should be fine.

.github/workflows/ci.yaml

@@ -37,8 +37,9 @@ jobs:
       - name: Show build.rs stderr
         shell: bash
         run: |
-          mapfile -d '' -t STDERR_FILES < <(find "./target/debug" -name stderr -print0 | grep -z bandwhich)
-          for FILE in "${STDERR_FILES[@]}"; do
+          # it's probably okay to assume no spaces?
+          STDERR_FILES=$(find "./target/debug" -name stderr | grep bandwhich)
+          for FILE in $STDERR_FILES; do
             echo "::group::$FILE"
             cat "$FILE"
             echo "::endgroup::"