PosixSourceAccessor: Support roots that are not directories

We have to support this for `fetchTree { type = "file" }` (and
probably other types of trees that can have a non-directory at the
root, like NARs).

src/libutil/posix-source-accessor.cc

@@ -30,6 +30,11 @@ std::filesystem::path PosixSourceAccessor::makeAbsPath(const CanonPath & path)
 {
     return root.empty()
         ? (std::filesystem::path { path.abs() })
+        : path.isRoot()
+        ? /* Don't append a slash for the root of the accessor, since
+             it can be a non-directory (e.g. in the case of `fetchTree
+             { type = "file" }`). */
+          root
         : root / path.rel();
 }