Fix debug I/O color detection for most targets

On POSIX, start.zig did not reset the `environ_initialized` field, which
prevented the environment variables from ever actually getting scanned.

On Windows, environment variable scanning is allocation-free, so it's
okay for `std.Io.Threaded.init_single_threaded` to use the global
environment block.

lib/std/Io/Threaded.zig

@@ -1671,20 +1671,23 @@ pub fn init(
 /// When initialized this way:
 /// * cancel requests have no effect.
 /// * `deinit` is safe, but unnecessary to call.
-pub const init_single_threaded: Threaded = .{
-    .allocator = .failing,
-    .stack_size = std.Thread.SpawnConfig.default_stack_size,
-    .async_limit = .nothing,
-    .cpu_count_error = null,
-    .concurrent_limit = .nothing,
-    .old_sig_io = undefined,
-    .old_sig_pipe = undefined,
-    .have_signal_handler = false,
-    .argv0 = .empty,
-    .environ_initialized = true,
-    .environ = .empty,
-    .worker_threads = .init(null),
-    .disable_memory_mapping = false,
+pub const init_single_threaded: Threaded = init: {
+    const env_block: process.Environ.Block = if (is_windows) .global else .empty;
+    break :init .{
+        .allocator = .failing,
+        .stack_size = std.Thread.SpawnConfig.default_stack_size,
+        .async_limit = .nothing,
+        .cpu_count_error = null,
+        .concurrent_limit = .nothing,
+        .old_sig_io = undefined,
+        .old_sig_pipe = undefined,
+        .have_signal_handler = false,
+        .argv0 = .empty,
+        .environ_initialized = env_block.isEmpty(),
+        .environ = .{ .process_environ = .{ .block = env_block } },
+        .worker_threads = .init(null),
+        .disable_memory_mapping = false,
+    };
 };
 
 var global_single_threaded_instance: Threaded = .init_single_threaded;

lib/std/start.zig

@@ -631,6 +631,7 @@ inline fn callMainWithArgs(argc: usize, argv: [*][*:0]u8, envp: [:null]?[*:0]u8)
     if (std.Options.debug_threaded_io) |t| {
         if (@sizeOf(std.Io.Threaded.Argv0) != 0) t.argv0.value = argv[0];
         t.environ = .{ .process_environ = .{ .block = env_block } };
+        t.environ_initialized = env_block.isEmpty();
     }
     std.Thread.maybeAttachSignalStack();
     std.debug.maybeEnableSegfaultHandler();