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:

curl: (23) Failed writing body (0 != 1381)

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

also demonstrated by a problem mentioned in the forum 3 years ago