Fix nix linting of submodules

Linting was breaking on including `sonata-system` as a submodule, due to
one of the vendored files from coremark in `sonata-system` containing a
non-UTF-8 copyright symbol in one of its markdown files. This broke
lychee linting.

In general, it doesn't make sense to lint submodules included under the
`third_party` directory. The exclude path contained `vendor`, which does
not exist, but not `third_party` - this is likely an error from copying
or a typo. This commit excludes that path from the lychee lint to avoid
these lints, fixing errors.

mypy was also running into python module naming conflicts with a
`test_runner` in both `sonata-software` and `sonata-system`, so a flag
has similarly been added to the `mypy` linting call to exclude the
`third_party` directory.

flake.nix

@@ -131,7 +131,7 @@
         text = ''
           ruff format --check .
           ruff check .
-          mypy .
+          mypy --exclude third_party .
         '';
       };
 

lychee.toml

@@ -4,6 +4,6 @@
 include_fragments = true
 exclude_path = [
   "./cheriot-rtos",
-  "./vendor",
+  "./third_party",
   "./build",
 ]