installing a local pack fails if git is not installed

[matko]

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Starting in 9.1.22, it has become necessary to have git installed to install a pack, even if the pack to be installed is already present locally.

I suspect this is due to https://github.com/SWI-Prolog/swipl-devel/blob/fcdbb296d354de12e492834d9fbe0fdd8d088b3e/library/prolog_pack.pl#L896.

It looks like link(true) would be a good way to not reach that branch, but unfortunately, that leads to errors down the line when a status.db is attempted to be written in the source directory, which might not be writable.

Maybe this could check for an explicit git(false) in the options before attempting to call git.

Initially reported here: NixOS/nixpkgs#336326