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/path.bash

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