cli: disallow impure derivations

we don't remove the entire feature in one go to make review easier.
impure derivations are rather unintrusive on their own, at least if
we compare them to dynamic or ca derivations in general, so we will
be done with this soon. as it stands impure derivations cannot work
without ca derivations, and those we *really* want to leave behind.

Change-Id: I4f01d8d758b2c85dcd6c3078304b5ee1b52f65b0

tests/unit/libexpr/error_traces.cc

@@ -1290,16 +1290,6 @@ namespace nix {
                       HintFmt("expected a Boolean but found %s: %s", "a string", "\"true\""),
                       HintFmt("while evaluating the attribute '__contentAddressed' of derivation 'foo'"));
 
-        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"out\"; __impure = \"true\"; }",
-                      TypeError,
-                      HintFmt("expected a Boolean but found %s: %s", "a string", "\"true\""),
-                      HintFmt("while evaluating the attribute '__impure' of derivation 'foo'"));
-
-        ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"out\"; __impure = \"true\"; }",
-                      TypeError,
-                      HintFmt("expected a Boolean but found %s: %s", "a string", "\"true\""),
-                      HintFmt("while evaluating the attribute '__impure' of derivation 'foo'"));
-
         ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"out\"; args = \"foo\"; }",
                       TypeError,
                       HintFmt("expected a list but found %s: %s", "a string", "\"foo\""),