nimblecache conflicts on multiuser system
It looks like nimble writes a directory `/tmp/nimblecache` on linux. 

'/tmp' is a shared folder between all users on a system, and if one user creates a folder, another won't be able to write to it, due to folder permissions. 

Thus, if user `a` runs nimble, then user `b` wants to run nimble, an error happens:

```
nimble test
nimble.nim(1115)         nimble
nimble.nim(1092)         doAction
nimscriptexecutor.nim(21) execHook
packageparser.nim(398)   getPkgInfoFromFile
packageparser.nim(349)   readPackageInfo
nimscriptsupport.nim(542) readPackageInfoFromNims
nimscriptsupport.nim(386) execScript
sysio.nim(406)           writeFile
system.nim(2836)         sysFatal
Error: unhandled exception: cannot open: /tmp/nimblecache/nimblepkg/nimscriptapi.nim [IOError]
```