Give a better error for download-ci-llvm if .xz is not supported

[jyn514]

Previously:

curl: (22) The requested URL returned error: 404
failed to run: curl -# -y 30 -Y 10 --connect-timeout 30 --retry 3 -Sf -o /tmp/tmp6ptXJV https://ci-artifacts.rust-lang.org/rustc-builds/99ad5a1a2824fea1ecf60068fd3636beae7ea2da/rust-dev-nightly-x86_64-unknown-linux-gnu.tar.gz

Now:

error: XZ support is required to download LLVM
help: consider disabling `download-ci-llvm` or using a different version of python
Build completed unsuccessfully in 0:00:00

Follow-up to #80435.

r? @Mark-Simulacrum

[jyn514]

It's possible to special case this so that it will work before 1.51, but since download-ci-llvm was only introduced a couple versions ago I don't think there's a need. The fix is easy enough - using python3 x.py instead works fine.

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.channel         := nightly
configure: rust.debug-assertions := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
Checking which error codes lack tests...
Found 435 error codes
Found 0 error codes with no tests
Done!
tidy error: /checkout/src/bootstrap/bootstrap.py:493: line longer than 100 chars
some tidy checks failed

command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "/checkout/obj/build"
expected success, got: exit code: 1

[Mark-Simulacrum]

Can we tell people a specific version? Maybe python 3 at least?

[jyn514]

Sure, I changed this to recommend python3.

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit b5019d6 has been approved by Mark-Simulacrum

[Mark-Simulacrum]

@bors p=1

[bors]

⌛ Testing commit b5019d6 with merge 417fe47...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 417fe47 to master...