`build.sh` fails to run (Gentoo Linux)

[ruby-R53]

System Details

• OS: Gentoo Linux, ~amd64 branch
• Thorium Version: 41d2f78

Problem
The "Build Thorium" step from BUILDING.md simply won't work. Running ./build.sh 16 yields the following:

Building Thorium for Linux...

siso ninja --offline -C out/thorium thorium_all -local_jobs=16
flag provided but not defined: -local_jobs
[...]
flag provided but not defined: -local_jobs

(full output: https://termbin.com/v5bs)

Then, without the jobs number argument:

Building Thorium for Linux...

Traceback (most recent call last):
  File "/home/efl/depot_tools/autoninja.py", line 707, in <module>
    sys.exit(main(sys.argv))
             ^^^^^^^^^^^^^^
  File "/home/efl/depot_tools/autoninja.py", line 693, in main
    exit_code = _main_inner(input_args, build_id, telemetry_cfg)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/efl/depot_tools/autoninja.py", line 314, in _main_inner
    j_value = input_args[index + 2]
              ~~~~~~~~~~^^^^^^^^^^^
IndexError: list index out of range

I've been trying to build this for a while, but this error always shows and doesn't seem to have been fixed.
I have all the dependencies already installed (they are the same ones required to build Chromium, right?), including depot_tools. But this looks more like an error in the code than anything else, or maybe my interpreter is broken? (Python 3.13.12)

[gz83]

Shouldn't we use emerge for Gentoo Linux?

[ruby-R53]

that's the neat part, no one has added Thorium to the repos there yet 🥀