Add instructions of how to use pre-vendored 'rustc-src'

[seqre]

This PR closes #110163.

I had to move the URL to the left, making it not aligned as it is three lines above, but the tidy check would yell at me otherwise. If that's not acceptable, I'd love some suggestions on how to make it better.

One question: in the original issue (#110163 (comment)), it was suggested to mention how to download specific commit tarballs; however, it was said it's not documented anywhere, so I did not include that yet. If there is a want to have that, I'd gladly amend the commit.

[rustbot]

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[seqre]

I have updated the code to reflect the discussion here. I've added an indent so the message stands out better in the output. The whole ERROR looks like this now:

ERROR: vendoring required, but vendor directory does not exist.
       Run `cargo vendor --sync ./src/tools/cargo/Cargo.toml --sync ./src/tools/rust-analyzer/Cargo.toml --sync ./compiler/rustc_codegen_cranelift/Cargo.toml --sync ./src/bootstrap/Cargo.toml ` to initialize the vendor directory.
       Alternatively, use the pre-vendored `rustc-src` dist component.
       To get a stable/beta/nightly version, download it from: 
       https://forge.rust-lang.org/infra/other-installation-methods.html#source-code
       To get a specific commit version, download it using the below URL,
       replacing <commit> with a specific commit checksum: 
       https://ci-artifacts.rust-lang.org/rustc-builds/<commit>/rustc-nightly-src.tar.xz
       Once you have the source downloaded, place the vendor directory
       from the archive in the root of the rust project.

@rustbot review

[onur-ozkan]

It would be great if we used the actual commit hash instead of the placeholder. I will still merge it because it's already an improvement over the current approach.

[onur-ozkan]

@bors r+ rollup

[bors]

📌 Commit 3cde0e8 has been approved by onur-ozkan

It is now in the queue for this repository.