std.Build.Cache: handle ENOENT on createFile race

There are no dir components, so you would think that this was
unreachable, however we have observed on macOS two processes racing
to do openat() with O_CREAT manifest in ENOENT.

Author: andrewrk

lib/std/Build/Cache.zig

@@ -439,7 +439,10 @@ pub const Manifest = struct {
                 self.manifest_file = manifest_file;
                 self.have_exclusive_lock = true;
             } else |err| switch (err) {
-                error.WouldBlock => {
+                // There are no dir components, so you would think that this was
+                // unreachable, however we have observed on macOS two processes racing
+                // to do openat() with O_CREAT manifest in ENOENT.
+                error.WouldBlock, error.FileNotFound => {
                     self.manifest_file = try self.cache.manifest_dir.openFile(&manifest_file_path, .{
                         .lock = .Shared,
                     });