Update exec.sh

Author: plengauer

tests/auto/exec.sh

@@ -2,8 +2,9 @@ set -e
 . /usr/bin/opentelemetry_shell.sh
 if [ "$OPEN_FD" = TRUE ]; then exec 3>&2; fi
 if [ "$SOURCE" = TRUE ]; then
-  echo 'exec echo' "$@" > source_exec.sh
-  . ./source_exec.sh
+  file="$(\mktemp)"
+  echo 'exec echo' "$@" > "$file"
+  . "$file"
 else
   exec echo "$@"
 fi