Parse the version from cargo pkgid with parameter expansion

[stepchowfun]

Unifies how the version is read from cargo pkgid on one form, parameter expansion:

PKGID="$(cargo pkgid)"
VERSION="${PKGID##*[#@:]}"

This strips everything through the last #, @, or :, leaving the version for every cargo pkgid format.

Replaced:

• .github/workflows/ci.yml: cut -d# -f2 | cut -d: -f2. It handled Cargo's old name:version format, but with current Cargo it returns name@version whenever the checkout directory's name differs from the package name.
• toast.yml (publish task): grep --extended-regexp --only-matching '[0-9.]+$'. It handled every format but truncated prerelease versions (1.2.3-beta.1 became .1).

Verified: with Cargo 1.99, cargo pkgid prints …/dir#1.2.3 when the directory matches the package name, …/dir#name@1.2.3 when it doesn't, and …#name@1.2.3-beta.1 for a prerelease. The expansion gave the right version in all three, while cut failed the last two and grep failed the prerelease. In this repository, I ran each changed line, extracted from the files, in Bash with set -euo pipefail (the CI step's shell, and the shell Toast tasks run in), and each produced the version cargo metadata reports. Both YAML files still parse.

This is the same change in all the Rust projects.

Status: Ready

Fixes: N/A