support multiple users using choosenim
currently only one user can use choosenim at a time due to permission problems in the /tmp directory.

steps to reproduce:

login as user1
`curl https://nim-lang.org/choosenim/init.sh -sSf | sh`
(success)

login as user2
`curl https://nim-lang.org/choosenim/init.sh -sSf | sh`
error:
```choosenim-init: Downloading choosenim-0.8.4_linux_amd64
curl: (23) Failed writing body (0 != 1381)
```

workaround:
user2 asks user1 to please delete all /tmp/choosenim-* directories.
