fix swapped logic for Windows

Remove `std.fs.deleteTree`. Callers instead should use
`std.fs.cwd().deleteTree`.

Add `std.fs.deleteTreeAbsolute` for when the caller has an absolute
path.

test/cli.zig

@@ -36,7 +36,7 @@ pub fn main() !void {
         testMissingOutputPath,
     };
     for (test_fns) |testFn| {
-        try fs.deleteTree(dir_path);
+        try fs.cwd().deleteTree(dir_path);
         try fs.cwd().makeDir(dir_path);
         try testFn(zig_exe, dir_path);
     }