Make CLI based fetch the default

[gitartpiano]

The libgit2 based git operations have some problems (with SSH).
There is a workaround for this problem: setting the CARGO_NET_GIT_FETCH_WITH_CLI environment variable to true.

If the CLI based git access has no known problems, then why not make CLI the default option?
This default could be overwritten by setting the CARGO_NET_GIT_FETCH_WITH_CLI environment variable to false, in case some users don't like the more sensible new default.

[Hirevo]

By default, Alexandrie already manages its crate index using the git command line utility instead of libgit2, as specified in the default alexandrie.toml file:

alexandrie/alexandrie.toml

Lines 20 to 22 in 1eb1b3c

	[index]
	type = "command-line"
	path = "crate-index"

The issue with a crate index configured using SSH is that the issue we mentioned arises on the users' machine (the consumers of the libraries hosted by the instance) when they perform cargo build or cargo search.
It is an issue happening outside of Alexandrie's reach.

Fixing the default can only be done within Cargo itself; here is a direct link to the libgit2 authentication handling in Cargo's source code.
This is the piece of code that, reportedly, does not always find the needed credentials to pull/push through SSH.
And by setting CARGO_NET_GIT_FETCH_WITH_CLI, we tell Cargo to not attempt to find the credentials and do things itself, but rather to let the system git utility to sort it out for us.

[gitartpiano]

Your explanations clarified for me.

I have also found out from cargo's https://doc.rust-lang.org/cargo/reference/config.html , that instead of the environment variable CARGO_NET_GIT_FETCH_WITH_CLI, I can also write this

[net]
git-fetch-with-cli = true   # use the `git` executable for git operations

into top level cargo config file:

Windows: %USERPROFILE%\.cargo\config.toml
Unix: $HOME/.cargo/config.toml

Since this is an easy workaround, there is no need for changing any defaults, I changed my mind.