style: Normalize variable expansions in libs.deprecated/*

These files stay deprecated and out of the sourcing path for now, but
apply the same ${var} notation cleanup as the rest of the repo where
it's a pure notation fix. basedir.bash is left untouched: its bug is
a broken escape, not a notation issue, and it mirrors the one already
removed from _helpers.bash.

libs.deprecated/prompt_command.bash

@@ -1,8 +1,8 @@
 #!/bin/bash
 
 _prompt_command_add() {
-    if [ -n "$1" ] && [[ ":$PROMPT_COMMAND:" != *":$1:"* ]]; then
-        PROMPT_COMMAND="${PROMPT_COMMAND:+"$PROMPT_COMMAND;"}$1"
+    if [ -n "$1" ] && [[ ":${PROMPT_COMMAND}:" != *":$1:"* ]]; then
+        PROMPT_COMMAND="${PROMPT_COMMAND:+"${PROMPT_COMMAND};"}$1"
     fi
 }