derived-path: refuse built derived path with a non-derivation base

Example: /nix/store/dr53sp25hyfsnzjpm8mh3r3y36vrw3ng-neovim-0.9.5^out

This is nonsensical since selecting outputs can only be done for a
buildable derivation, not for a realised store path. The build worker
side of things ends up crashing with an assertion when trying to handle
such malformed paths.

Change-Id: Ia3587c71fe3da5bea45d4e506e1be4dd62291ddf

tests/unit/libstore/derived-path.cc

@@ -77,6 +77,15 @@ TEST_F(DerivedPathTest, built_built_xp) {
         MissingExperimentalFeature);
 }
 
+/**
+ * Built paths with a non-derivation base should fail parsing.
+ */
+TEST_F(DerivedPathTest, non_derivation_base) {
+    ASSERT_THROW(
+        DerivedPath::parse(*store, "/nix/store/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x^foo"),
+        InvalidPath);
+}
+
 #ifndef COVERAGE
 
 RC_GTEST_FIXTURE_PROP(