smallpt unexpected output on Darwin

I ran nofib and got this error:

cd real/veritas; valgrind --tool=cachegrind --cachegrind-out-file=/Users/nineonine/Projects/ghc3/nofib/_make/base/real/veritas/Main.cachegrind.result.1 --log-file=cachegrind.log /Users/nineonine/Projects/ghc3/nofib/_make/base/real/veritas/Main 400 +RTS --machine-readable -t/Users/nineonine/Projects/ghc3/nofib/_make/base/real/veritas/Main.stats.1
nofib-run: Error when running Shake build system:
  at want, called at runner/Main.hs:185:23 in main:Main
* Depends on: _make/base/cachegrind.results.tsv
  at need, called at runner/Main.hs:204:11 in main:Main
* Depends on: _make/base/real/smallpt/Main.cachegrind.results.tsv
* Raised the exception:
user error (Benchmark failed:real/smallpt unexpected stdout:^)

I invoked nofib with this command:

cabal v2-run -- nofib-run --compiler ~/Projects/ghc3/_build/stage1/bin/ghc --output base --cachegrind --speed Fast --clean --threads 4 --head

Contents of stdout.expected:

42517.5950581666

Contents of stdout.actual:

42517.59505816911

stdout snapshot for this test was introduced here: 24a179b1

CC @bgamari @mpickering