std.ChildProcess: fix EBADF when explicitly passing cwd as working directory

[LordMZTE]

closes #9688

This fixes a bug where ChildProcess.spawn would try to call fchdir with AT_FDCWD causing an EBADF error to occur when ChildProcess.cwd_dir is set to std.fs.cwd().

[squeek502]

Worth noting that this is not the only place AT.FDCWD can cause problems (e.g. I know os.fstat has the same issue but the fix there is much less clear), and I'm unsure about exactly where this type of thing should be accounted for. For example, this same issue could be fixed by doing

if (dirfd == AT.FDCWD) return;

in std.os.fchdir instead. Note that this alternate fix would also avoid EBADF when doing something like:

test {
    const cwd = fs.cwd();
    try cwd.setAsCwd();
}

whereas the implementation in this PR would still hit EBADF in that case.

[LordMZTE]

Right. Let's see if the maintainers would prefer this fix.

[andrewrk]

Let's please do the fix suggested by @squeek502

[LordMZTE]

Got it, I'll open this as a new PR as this one is getting pretty outdated and contains a redundant commit.