Python: Fixup threat-models for os.environ.get()

Since using `.DictionaryElementAny` doesn't actually do a store on the
source, (so we can later follow any dict read-steps).

I added the ensure_tainted steps to highlight that the result of the
WHOLE expression ends up "tainted", and that we don't just mark
`os.environ` as the source without further flow.

python/ql/test/experimental/meta/InlineTaintTest.qll

@@ -15,6 +15,7 @@ import semmle.python.dataflow.new.TaintTracking
 import semmle.python.dataflow.new.RemoteFlowSources
 import TestUtilities.InlineExpectationsTest
 private import semmle.python.dataflow.new.internal.PrintNode
+private import semmle.python.Concepts
 
 DataFlow::Node shouldBeTainted() {
   exists(DataFlow::CallCfgNode call |
@@ -45,7 +46,7 @@ module Conf {
         source.(DataFlow::CfgNode).getNode() = call.getAnArg()
       )
       or
-      source instanceof RemoteFlowSource
+      source instanceof ThreatModelSource
     }
 
     predicate isSink(DataFlow::Node sink) {