Provide options for V2 cli to override the default bundle_url

[Sinofine]

When we use tectonic -X new or tectonic -X init to initiate projects, the output is as follows.

note: "version 2" Tectonic command-line interface activated
note: creating new document in directory `xxxx`
note: connecting to https://relay.fullyjustified.net/default_bundle.tar
note: resolved to https://data1.fullyjustified.net/tlextras-2020.0r0.tar

It seems that the url is hard coded in

tectonic/crates/bundles/src/lib.rs

Lines 114 to 122 in 8bf8da7

	pub fn get_fallback_bundle_url(format_version: u32) -> String {
	// Format version 32 (TeXLive 2021) was when we introduced versioning to the
	// URL.
	if format_version < 32 {
	"https://relay.fullyjustified.net/default_bundle.tar".to_owned()
	} else {
	format!("https://relay.fullyjustified.net/default_bundle_v{format_version}.tar")
	}
	}

and the config is default using that source

tectonic/src/config.rs

Lines 194 to 202 in 8bf8da7

	impl Default for PersistentConfig {
	fn default() -> Self {
	let url = tectonic_bundles::get_fallback_bundle_url(tectonic_engine_xetex::FORMAT_SERIAL);
	PersistentConfig {
	default_bundles: vec![BundleInfo { url }],
	}
	}
	}

The source listed above is acceptable for daily usage, but for users in an internal network without access to internet, they may use mirrors or local copy for bundles. However, the command tectonic -X new or tectonic -X init themselves are default using the hard-coded bundle url to complete initiation, which is meant if the default url is not accessible then the initiation won't be accomplished (usually we waits for minutes and then it throws connection errors and fails).

In the V1 cli, it provides --web-bundle to override the default, but in the newer V2 cli, the option is absent. If we provide the option for tectonic -X new and tectonic -X init, it may be nicer for people unable to access the default bundle url.

[pkgw]

Yes, this is a good point. This should be a fairly straightforward improvement for someone new to the project to implement, I'd hope, so I'll tag as such!

[bryango]

I came up with a fix for this in #1132. Please see if it's okay!