Return empty string instead of throwing when getpwuid_r fails unexpectedly

[HarnageaGabriel]

Fixes #119216

Problem

GetUserNameFromPasswd's XML doc already documents that it returns an empty string on failure. That contract already holds when /etc/passwd is readable but the current UID isn't present (getpwuid_r returns -1/ENOENT-equivalent → TryGetUserNameFromPasswd returns null).

But when getpwuid_r fails for any other reason — notably when /etc/passwd itself doesn't exist, as can happen in restricted sandboxes/containers — TryGetUserNameFromPasswd threw an IOException instead. That exception propagates up through Environment.UserName and crashes the calling process (e.g. dotnet build, dotnet new) instead of degrading gracefully.

Repro (from the issue)

docker run -it ubuntu:24.04
apt update && apt install -y dotnet-sdk-8.0
rm /etc/passwd
cd /root
dotnet new create console

Fix

src/libraries/Common/src/Interop/Unix/System.Native/Interop.GetPwUid.cs: in TryGetUserNameFromPasswd, the fallback branch after the ERANGE (buffer-too-small) check no longer throws — it now sets username = null and returns true, same as the "current user not found" path, so GetUserNameFromPasswd returns string.Empty instead of propagating the exception. The ERANGE retry-with-larger-buffer behavior is unchanged.

Also removed the now-unused using System.IO;.

Testing

This is a small interop fix in src/libraries/Common (shared source, no dedicated isolated test project); given the size of the dotnet/runtime repo I did not run a full runtime build. I verified the change by inspecting the diff and confirming no other caller relies on the previously-thrown IOException (checked Environment.UserName in Environment.Unix.cs, the only caller).

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[HarnageaGabriel]

@dotnet-policy-service agree

[dotnet-policy-service]

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