disambiguate ENV_VAR_REGEX in rex.py / utils/formatting.py

[nerdvegas]

There are two very similar but separate consts in these sourcefiles.

The instance in formatting.py isn't ideal, it would be better served to match the value in rex.py, and the manual parsing of curly braces can be removed.

These two sourcepaths should probably share the same regex instance. However, we first need to determine if ENV_VAR_REGEX needs to be overridable in shells (see #1199). If it does not, we should remove the class attrib from ActionInterpreter (in rex) altogether and just use ENV_VAR_REGEX imported from formatting.py, to avoid confusion.