chore(nix): unify Linux cross-compilation toolchains - #3242

Replace the separate GNU and musl development shells with one shell that
provides explicit toolchains for x86_64 and aarch64 Linux, plus native
Darwin on macOS. Cargo selects the compiler, assembler, archiver, and
linker through target-specific environment variables.

Build GNU targets against a glibc 2.28 sysroot with static GCC runtimes
and use the musl toolchains for static Linux executables. Build Z3 and
AWS-LC with each target's stdenv and expose AWS-LC libraries and Rust
bindings through its target-specific system directory.

Keep Darwin system libraries on the unprocessed Apple SDK and prevent
the Rust toolchain from propagating replacement libraries into the shell.
Include the compiler and libc fixes needed for Darwin-to-Linux builds.

Share dependency and environment wiring through mkToolchain, keep compiler
wrappers in the Linux and Darwin modules, and group the pinned GNU build
environment under glibc-2.28. Document the toolchain boundaries in the build
architecture overview.

Validation: actionlint and nix fmt pass. The toolchain refactor preserves
the shell derivations for x86_64 Linux, aarch64 Linux, and aarch64 Darwin.

Signed-off-by: Simon Scatton <sscatton@nvidia.com>

Remove the dev-shell input and its matrix and workflow plumbing.
Use the host default shell for builds and cache hashing.

Signed-off-by: Simon Scatton <sscatton@nvidia.com>

Use target-specific artifact paths and rely on the Nix toolchains for
linkage. Remove post-link rewriting, platform linkage checks, and the
unused interpreter input. Update the build architecture documentation.

Signed-off-by: Simon Scatton <sscatton@nvidia.com>

Use a pinned historical Nixpkgs recipe for glibc 2.28 and rebuild GCC 15
against it instead of maintaining separate runtime builds. Keep only
compatibility adjustments needed by the current build tools.

Assemble the sysroot from glibc outputs and static native libraries. Use
standard ELF interpreters and resolve Rust's explicit gcc_s dependency
through the static GCC archives.

Allow 90 minutes for Rust branch checks to accommodate cold toolchain
builds. Validate the Linux release matrix locally; Darwin remains for CI.

Signed-off-by: Simon Scatton <sscatton@nvidia.com>