Copy task: make ERROR_ACCESS_DENIED fail-fast Windows-only to fix non-Windows retry resilience

src/Tasks.UnitTests/Copy_Tests.cs

@@ -753,7 +753,13 @@ public void DoNotNormallyCopyOverReadOnlyFile(bool isUseHardLinks, bool isUseSym
                 string destinationContent = File.ReadAllText(destination);
                 Assert.Equal("This is a destination file.", destinationContent);
 
-                ((MockEngine)t.BuildEngine).AssertLogDoesntContain("MSB3026"); // did not do retries as it was r/o
+                if (NativeMethodsShared.IsWindows)
+                {
+                    // On Windows, ERROR_ACCESS_DENIED is not retried (it's a real ACL or r/o bit issue).
+                    ((MockEngine)t.BuildEngine).AssertLogDoesntContain("MSB3026");
+                }
+                // On non-Windows, access denied can be a transient lock (e.g. macOS CoW filesystem),
+                // so we retry; retries will ultimately fail for a genuinely read-only file.
             }
             finally
             {

src/Tasks/Copy.cs

@@ -1033,7 +1033,10 @@ private bool DoCopyWithRetries(FileState sourceFileState, FileState destinationF
                                 // to a failure to reset the readonly bit properly, in which case retrying will succeed.  This seems to be
                                 // a pretty edge scenario, but since some of our internal builds appear to be hitting it, provide a secret
                                 // environment variable to allow overriding the default behavior and forcing retries in this circumstance as well.
-                                if (!s_alwaysRetryCopy)
+                                // On non-Windows platforms (e.g. macOS copy-on-write filesystems), access denied can also indicate a
+                                // transient lock conflict (e.g. EACCES from a concurrent clonefile/flock), so we only skip retries for
+                                // access denied on Windows.
+                                if (!s_alwaysRetryCopy && NativeMethodsShared.IsWindows)
                                 {
                                     throw;
                                 }