std.Build.Cache.addDepFile fails if the dep file contains directories

[akarpovskii]

Zig Version

0.14.0-dev.3445+6c3cbb0c8

Steps to Reproduce and Observed Behavior

1. zig init an empty project
2. Create a "./depinfo.d" file with the following content:

   a: src

3. Add the following to the build.zig:

   const cmd = b.addSystemCommand(&.{ "cp", "depinfo.d" });
   _ = cmd.addDepFileOutputArg("depinfo.d");
   run_cmd.step.dependOn(&cmd.step);

4. ------> zig build run fails with error: IsDir

Expected Behavior

zig build run finishes successfully or reports a clear error

This is somewhat related to #20099 and #21912