Install xargo using CI dictated cargo version if available

[jackcmay]

Problem

The SDK install script attempts to install xargo which is required to build Rust BPF programs. The script uses whatever version of cargo is installed on the host machine to do so. in CI the version of cargo is always explicit and the machine's default toolchain is not updated, consequently the CI machines default version is very old. This old version does not support installing the latest version of xargo.

Summary of Changes

• Use the CI version of cargo if the CI defined cargo version environment variable is defined (rust_stable.
• cargo install now installs the latest version of a package so remove the use of cargo install-update

Fixes #

[jstarry]

FYI looks like cargo install xargo will now update xargo to the latest version and will do nothing if it's already installed. This was released as part of 1.41.0: rust-lang/cargo#7560 So we can rip out cargo-update

[jackcmay]

@jstarry Cool, I'll both set the stable version as the default toolchain and rip out update and replace with cargo install

[jstarry]

@jackcmay, @mvines just added a note in #devops about setting the default toolchain being disruptive for devs, so we should be careful about that. Maybe we could change the command to cargo +stable to avoid that?

[codecov]

Codecov Report

Merging #9068 into master will increase coverage by <.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #9068     +/-   ##
========================================
+ Coverage    80.4%   80.4%   +<.1%     
========================================
  Files         268     268             
  Lines       58796   58796             
========================================
+ Hits        47320   47326      +6     
+ Misses      11476   11470      -6

[jackcmay]

Agreed, not going to set default toolchain

[jackcmay]

@mvines know any cool bash tricks to improve this?

[mvines]

I'd just go with a simple approach:

Suggested change

	cargo +"${rust_stable:-}" install xargo
	if [[ -n $rust_stable ]]; then
	cargo +"$rust_stable" install xargo
	else
	cargo install xargo
	fi

[jackcmay]

shellcheck does not like this since it can't tell if rust_stable is defined (rust_stable should probably be in all caps) and would rather not have to clutter up with a spellcheck "allow" statement