Fix race conditions in task host path resolution

[AR-May]

Fixes #13484

Context

Multithreaded (/mt) builds that launch out-of-process task hosts crash with ArgumentNullException: Value cannot be null. Parameter name: path2 in Path.Combine when multiple RequestBuilder threads simultaneously resolve the task host executable name.

Root cause

GetTaskHostNameFromHostContext in NodeProviderOutOfProcTaskHost.cs has an unsafe read-modify-write sequence on the static field s_msbuildName:

if (string.IsNullOrEmpty(s_msbuildName))            // (1) check
{
    s_msbuildName = Environment.GetEnvironmentVariable("MSBUILD_EXE_NAME"); // (2) write — may be null
    if (!string.IsNullOrEmpty(s_msbuildName))
    {
        return s_msbuildName;
    }
    s_msbuildName = Constants.MSBuildExecutableName;  // (3) write — always non-null
}
return s_msbuildName;                                // (4) read

When MSBUILD_EXE_NAME is not set, step (2) writes null into s_msbuildName. Between steps (2) and (3), another thread can read s_msbuildName at step (4)** and get null.

The returned null is passed as toolName into Path.Combine(basePath, toolName), which throws ArgumentNullException on .NET Framework (where Path.Combine does not accept null arguments).

Changes Made

Since all computations are deterministic (environment variables + constants), we use the local-variable snapshot pattern
— read the static into a local, compute if null, write back, return the local. This is lock-free and safe because reference writes are atomic in .NET. Applied the same fix to similar patterns in nearby code in NodeProviderOutOfProcTaskHost.cs and CurrentHost.cs

Testing

No extra testing. There is no stable local repro.