fix(variable): correct email validation regex

Fixed malformed email validation regex that was matching literal
backslash-s characters instead of whitespace. Changed from
r"^[^@\\s]+@[^@\\s]+\\.[^@\\s]+$" to
r"^[^@\s]+@[^@\s]+\.[^@\s]+$"

Fixes #1481

cli/core/template/variable.py

@@ -12,7 +12,7 @@ logger = logging.getLogger(__name__)
 
 TRUE_VALUES = {"true", "1", "yes", "on"}
 FALSE_VALUES = {"false", "0", "no", "off"}
-EMAIL_REGEX = re.compile(r"^[^@\\s]+@[^@\\s]+\\.[^@\\s]+$")
+EMAIL_REGEX = re.compile(r"^[^@\s]+@[^@\s]+\.[^@\s]+$")
 
 
 class Variable: