keepkey: bump nanopb to 0.4.9.1

CI fails with:

```
Traceback (most recent call last):
  File "/home/runner/work/HWI/HWI/test/work/keepkey-firmware/nanopb/generator/nanopb_generator.py", line 1816, in <module>
    main_plugin()
  File "/home/runner/work/HWI/HWI/test/work/keepkey-firmware/nanopb/generator/nanopb_generator.py", line 1796, in main_plugin
    other_files[fdesc.name] = parse_file(fdesc.name, fdesc, options)
                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/HWI/HWI/test/work/keepkey-firmware/nanopb/generator/nanopb_generator.py", line 1628, in parse_file
    Globals.separate_options = read_options_file(open(optfilename, "rU"))
                                                 ^^^^^^^^^^^^^^^^^^^^^^^
ValueError: invalid mode: 'rU'
--nanopb_out: protoc-gen-nanopb: Plugin failed with status code 1.
```

https://github.com/bitcoin-core/HWI/actions/runs/16440824447/job/46462763445

This was changed in:
nanopb/nanopb@01e9186

That's been in the releases since 0.4.6.

Author: Sjors

test/setup_environment.sh

@@ -235,7 +235,7 @@ if [[ -n ${build_keepkey} ]]; then
     # Build the simulator. This is cached, but it is also fast
     if [ "$keepkey_setup_needed" == true ] ; then
         git clean -ffdx
-        git clone https://github.com/nanopb/nanopb.git -b nanopb-0.3.9.4
+        git clone https://github.com/nanopb/nanopb.git -b nanopb-0.4.9.1
     fi
     cd nanopb/generator/proto
     make