Probably since macOS Runner Image Version 20251020.XXXX, spawned
processes initialize `TMPDIR` environment variable under the hood.

And `LD_PRELOAD` is set to `PRELOADENV` on Linux.

We need to keep RUBY_FREE_AT_EXIT in these tests.

On systems where the Encoding.default_internal defaults to US-ASCII instead
of UTF-8, some tests using assert_raise_with_message can fail since it no
longer changes Encoding.default_internal in 79f5202.

This tests explicitly uses EnvUtil.with_default_internal on systems where
these tests fail.

See: 58bc97628c1

getpwnam(3) says the same thing. I got ENOENT in my Linux environment.

  1) Failure:
TestProcess#test_uid_from_name [/home/k0kubun/src/github.com/ruby/ruby/test/ruby/test_process.rb:1685]:
Exception(ArgumentError) with message matches to /\u{4e0d 5b58 5728}/.
[ArgumentError] exception expected, not #<Errno::ENOENT: No such file or directory - getpwnam_r>.

```
  1) Failure:
TestProcess#test_warmup_frees_pages [/tmp/ruby/src/trunk-random1/test/ruby/test_process.rb:2772]:
<164348> expected but was
<165985>.
```

can someone investigate it?

We no longer execute those files with Solaris platforms.