CLI nags to update with brew despite being installed with Nix

[cedws]

I have the CLI installed via Nix but says to update the Brew package:

A new release of infisical is available: 0.41.90 -> 0.43.84

To update, run: brew update && brew upgrade infisical

[BenyD]

The Linux branch of getUpdateInstructions never inspects execPath outside the npm check, so it falls through to getLinuxPackageManager(), which only probes which package managers exist on the machine. A Nix install on Debian or Ubuntu gets told to run apt-get install infisical for a binary in the immutable store.

The macOS path is already handled, there's a passing darwin nix returns empty case in the tests.

Opened #344 for the Linux side, mirroring the darwin behaviour so Nix returns no instruction rather than a wrong one. Homebrew-on-Linux had the same gap, since /home/linuxbrew/.linuxbrew/ doesn't match the /homebrew/ substring darwin checks for, so that's covered too.