Skip the pypy3 sample environment test on Darwin

[thomasjm]

sample-environments (macOS) is still red on main after #102, with the same 4 failures:

pypy3.nix
    readCreateProcessWithLogging: nix "build" ".#sample_environment_pypy3" "--no-link" "--json" (exit 1): failed
4 failed of 151

#102 gated the pypy3 sample environment out of the Darwin flake outputs, since PyPy's bootstrap
interpreter doesn't build there. But this suite enumerates the sample_environments/*.nix files
from disk via Template Haskell and builds .#sample_environment_<name> for each, so on Darwin it
now asks for an attribute that doesn't exist. Same failure, different cause — my miss in #102.

The file list is filtered against what the platform actually provides, mirroring the gating in
sample_environments.nix.

Testing

pypy3.nix still runs and passes under --sample-environments on Linux, so only Darwin is affected.

Note

sample_environments.nix also drops julia110, julia111, mega and python3{,11,12,13} on
aarch64-linux (issue #96). Those would hit this same problem, but the sample-environments job only
runs on x86_64 Linux and macOS, so it isn't reachable today. If that job ever runs on aarch64-linux,
unavailableHere needs the corresponding entries — or the list should come from the flake rather
than the filesystem.