fix sandbox detection to exclude directories

[thalesmg]

the previous detection of nix sandboxes was incorrectly considering directories that ended in -nix as a sandbox directory (it was searching for .nix without escaping the dot).

also, even fixing the regex, if one had a directory ending in .nix (for example: haskell.nix for IOHK's repo), the parent directory would be considered the sandbox directory.

therefore, here, we fix the regex to escape the dot and to also reject directories that end in .nix from the search.

Examples:

The examples below use the following test structure:

ͳ pwd
/home/thales/tmp/test-nix-sandbox
ͳ tree
.
├── current-project
│   └── current.buffer
├── test-nix
│   └── some.file
└── test.nix
    └── some.file

3 directories, 3 files

Results of (nix-current-sandbox)

• Before the changes: "/home/thales/tmp/test-nix-sandbox/"
• Fixing just the regex without test.nix dir: nil
• Fixing just the regex: "/home/thales/tmp/test-nix-sandbox/"
• Rejecting directories: nil