cdr: Fix 'core show channel' CDR variable truncation.

When the new Bridging API was implemented, the workspace variable
changed to a malloc'd string, causing sizeof() to always be 8 (char).

Revert back to stored on stack string for workspace.

ASTERISK-25090 #close

Change-Id: I51e610ae87371df771ce7693a955510efb90f8f7