Unicode-outputting integration tests fail with --nix

### General summary/comments (optional)
`new-package-names` and `1337-unicode-everywhere` fail with `stack --nix`

### Steps to reproduce
` stack --nix test --flag stack:integration-tests stack:stack-integration-test`

### Expected
Passing test

### Actual

` Main.hs: /home/shana/programming/stack/.stack-work/install/x86_64-linux-nix/lts-8.5/8.0.2/bin/stack: recoverEncode: invalid argument (invalid character)`
` cabal file path /run/user/1000/stack-integration-1337-unicode-everywhere12842/stack-integration-test: : commitBuffer: invalid argument (invalid character) `

Might want to set locale &c. in `shell.nix` or similar such fix.

### Stack version
`4abb06db83bda2103255c8812989520d17be332e`

### Method of installation
hacking in repo