Handle the missing buildroot in rpm 4.20 gracefully

Fall back to another way of determining potential buildroot locations
as 4.20 removed the use of buildroot.

rpmlint/checks/BuildRootAndDateCheck.py

@@ -17,7 +17,7 @@ def __init__(self, config, output):
         super().__init__(config, output, r'.*')
         self.looksliketime = re.compile('(2[0-3]|[01]?[0-9]):([0-5]?[0-9]):([0-5]?[0-9])')
         self.istoday = re.compile(time.strftime('%b %e %Y'))
-        self.prepare_regex(rpm.expandMacro('%buildroot'))
+        self.prepare_regex(rpm.expandMacro('%{?buildroot}') or '^/.*/BUILDROOT/')
 
     def prepare_regex(self, buildroot):
         for m in ('name', 'version', 'release', 'NAME', 'VERSION', 'RELEASE'):