build: make PrintStep thread safe

Update PrintStep to always printing to stderr, using std.debug.print, so
that the message is written atomically.

Note that currently it is not an issue, since PrintStep prints the
message before everything else.

build.zig

@@ -557,7 +557,7 @@ pub fn build(b: *Build) !void {
     const use_healed = b.option(bool, "healed", "Run exercises from patches/healed") orelse false;
     const exno: ?usize = b.option(usize, "n", "Select exercise");
 
-    const header_step = PrintStep.create(b, logo, std.io.getStdErr());
+    const header_step = PrintStep.create(b, logo);
 
     if (exno) |n| {
         if (n == 0 or n > exercises.len - 1) {
@@ -861,13 +861,12 @@ const ZiglingStep = struct {
     }
 };
 
-// Print a message to a file.
+// Print a message to stderr.
 const PrintStep = struct {
     step: Step,
     message: []const u8,
-    file: std.fs.File,
 
-    pub fn create(owner: *Build, message: []const u8, file: std.fs.File) *PrintStep {
+    pub fn create(owner: *Build, message: []const u8) *PrintStep {
         const self = owner.allocator.create(PrintStep) catch @panic("OOM");
         self.* = .{
             .step = Step.init(.{
@@ -877,7 +876,6 @@ const PrintStep = struct {
                 .makeFn = make,
             }),
             .message = message,
-            .file = file,
         };
 
         return self;
@@ -887,7 +885,7 @@ const PrintStep = struct {
         _ = prog_node;
         const p = @fieldParentPtr(PrintStep, "step", step);
 
-        try p.file.writeAll(p.message);
+        print("{s}", .{p.message});
     }
 };