Add FreeBSD-specific logic to pal::get_default_installation_dir_for_arch

[arrowd]

The full problem context: https://reviews.freebsd.org/D44560

This change tries to address it for the native FreeBSD case. The Linuxulator case can hopefully be fixed by installing /compat/linux/etc/dotnet/install_location.

@sec @Thefrank can you look into this?

[arrowd]

@dotnet-policy-service agree

[Thefrank]

LGTM

As an aside: /etc/dotnet/install_location file did also work for FreeBSD. I can only find documentation of it in design docs: https://github.com/dotnet/designs/blob/main/accepted/2020/install-locations.md so this may have changed.

[arrowd]

On FreeBSD the /etc is reserved for the base system usage. Third-party software should use a prefix path called LOCALBASE which defaults to /usr/local.

[Thefrank]

Fair point. That file would not make sense for a FreeBSD port.

oooh! That differential also has a working powershell port from "zirias" which means I can close out 3 upstream PRs for getting FreeBSD added to the 3 powershell repos. Happy to see someone get that over the finish line

[sec]

@arrowd LGTM. IIRC setting DOTNET_ROOT also works, as I use it on my machines, but your fix is more elegant.

[Thefrank]

Looking at this and according to hier(7), share is for architecture-independent files. I am not sure this location is a good fit for dotnet, however this specific file is actually arch agnostic as any arch of dotnet will look for it. As you are the port maintainer, I will defer to you where you want the files. hier(7) is more guidance than law as per the notes.

[arrowd]

Bump. Can we get this in?

[agocke]

@elinor-fung for review

[elinor-fung]

Apologies for the delay. Thanks, @arrowd!