nix: fix detecting install type in Docker

Otherwise build fails in a Docker container for Desktop:
```
+ make status-go
Building: status-go
Unknown Nix installtion type!
Unknown Nix installtion type!
make[1]: *** [Makefile:223: statusgo-shared-library] Error 1
make: *** [Makefile:432: vendor/status-go/build/bin/libstatus.so] Error 2
```

Signed-off-by: Jakub Sokołowski <jakub@status.im>

nix/scripts/lib.sh

@@ -27,6 +27,7 @@ nix_install_type() {
     if [[ "$(os_name)" =~ NixOS ]]; then
         echo "nixos"
     else
+        USER=$(id -un) # Missing in Docker.
         case "${NIX_STORE_DIR_GROUP}" in
             "nixbld")   echo "multi";;
             "30000")    echo "multi";;