Allow file targets in native_tool_toolchain() (#1117)

Some users might want to specify simple files (such as pre-built tools)
in the target attribute of native_tool_toolchain.

Co-authored-by: UebelAndre <github@uebelandre.com>

toolchains/native_tools/native_tools_toolchain.bzl

@@ -86,6 +86,7 @@ native_tool_toolchain = rule(
                 "If the tool is built as part of the build, the corresponding build target, " +
                 "which should produce the tree artifact with the binary to call."
             ),
+            allow_files = True,
         ),
     },
     incompatible_use_toolchain_transition = True,