Support linux-riscv64 and migrate recipe to v1

[wolfv]

This supersedes and includes the bot migration PR #40: #40

Changes:

• retains the linux-riscv64 migration configuration;
• converts meta.yaml to v1 recipe.yaml;
• switches the feedstock to rattler-build and pixi;
• uses extensionless platform-specific build scripts;
• replaces file checks with package-content tests for the executable, man page, and shell completions;
• explicitly forces the bundled static PCRE2 build to prevent host-library contamination;
• explicitly links the oldest Linux sysroot startup helpers required by the current Rust toolchain.

Local validation:

• conda-smithy lint --conda-forge .
• successful complete linux-64 build and tests
• confirmed the resulting ELF dependencies match the existing package
• all variants render successfully with current conda-smithy.

The RISC-V job remains externally blocked because rust_linux-riscv64 has not yet been published on conda-forge.

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/recipe.yaml) and found it was in an excellent condition.

[luhenry]

Depends on conda-forge/rust-feedstock#297 and conda-forge/rust-activation-feedstock#104