mktemp not working to make hidden temp file

Using GNU mktemp this works fine:
```
$ mktemp --version | head -n 2
mktemp (GNU coreutils) 9.7
Packaged by Ubuntu (9.7-3ubuntu2)
$ mktemp /tmp/.XXXXXXXX
/tmp/.WuCzSrUQ
```

Using uutils fails:
```
$ /usr/lib/cargo/bin/coreutils/mktemp --version
mktemp (uutils coreutils) 0.8.0
$ /usr/lib/cargo/bin/coreutils/mktemp /tmp/.XXXXXXXX
mktemp: Permission denied (os error 13) at path "/tmpZsls5hXb"
```
