parallel make in fuzzers breaks with assertion error

[gsomlo]

I'm trying to reproduce the steps required to run the fuzzers, starting with artix7. After installing vivado and prjxray, I followed the "quickstart guide" steps all the way to Step 8 (option 2). If I simply run single-threaded make under the fuzzers folder, things seem to work, but if I try make -j$(nproc), I get the following error:

...
2024-08-12T15:01:49 - xc7a100tfgg676-1/073-get_counts            -    0s: Running make -C /home/foobar/prjxray/fuzzers/073-get_counts run (with MAKEFLAGS=' -j8 --jobserver-auth=fifo:/tmp/GMfifo8785')
Traceback (most recent call last):
  File "/home/foobar/prjxray/fuzzers/.//run_fuzzer.py", line 692, in <module>
    sys.exit(main(sys.argv))
             ^^^^^^^^^^^^^^
  File "/home/foobar/prjxray/fuzzers/.//run_fuzzer.py", line 460, in main
    exit_code = run_fuzzer(
                ^^^^^^^^^^^
  File "/home/foobar/prjxray/fuzzers/.//run_fuzzer.py", line 540, in run_fuzzer
Traceback (most recent call last):
  File "/home/foobar/prjxray/fuzzers/.//run_fuzzer.py", line 692, in <module>
    assert job_re, make_flags
           ^^^^^^
AssertionError:  -j8 --jobserver-auth=fifo:/tmp/GMfifo8785
    sys.exit(main(sys.argv))
             ^^^^^^^^^^^^^^
  File "/home/foobar/prjxray/fuzzers/.//run_fuzzer.py", line 460, in main
    exit_code = run_fuzzer(
                ^^^^^^^^^^^
  File "/home/foobar/prjxray/fuzzers/.//run_fuzzer.py", line 540, in run_fuzzer
    assert job_re, make_flags
           ^^^^^^
AssertionError:  -j8 --jobserver-auth=fifo:/tmp/GMfifo8785
make: *** [Makefile:178: 072-ordered_wires/run.xc7a100tfgg676-1.ok] Error 1
make: *** Waiting for unfinished jobs....
make: *** [Makefile:181: 075-pins/run.xc7a100tfgg676-1.ok] Error 1
Traceback (most recent call last):
  File "/home/foobar/prjxray/fuzzers/.//run_fuzzer.py", line 692, in <module>
    sys.exit(main(sys.argv))
             ^^^^^^^^^^^^^^
  File "/home/foobar/prjxray/fuzzers/.//run_fuzzer.py", line 460, in main
    exit_code = run_fuzzer(
                ^^^^^^^^^^^
  File "/home/foobar/prjxray/fuzzers/.//run_fuzzer.py", line 540, in run_fuzzer
    assert job_re, make_flags
           ^^^^^^
AssertionError:  -j8 --jobserver-auth=fifo:/tmp/GMfifo8785
make: *** [Makefile:96: 005-tilegrid/run.xc7a100tfgg676-1.ok] Error 1
Traceback (most recent call last):
  File "/home/foobar/prjxray/fuzzers/.//run_fuzzer.py", line 692, in <module>
    sys.exit(main(sys.argv))
             ^^^^^^^^^^^^^^
  File "/home/foobar/prjxray/fuzzers/.//run_fuzzer.py", line 460, in main
    exit_code = run_fuzzer(
                ^^^^^^^^^^^
  File "/home/foobar/prjxray/fuzzers/.//run_fuzzer.py", line 540, in run_fuzzer
    assert job_re, make_flags
           ^^^^^^
AssertionError:  -j8 --jobserver-auth=fifo:/tmp/GMfifo8785
make: *** [Makefile:179: 073-get_counts/run.xc7a100tfgg676-1.ok] Error 1

Any clue as to what I may be missing much appreciated!