ci: fix riscv64 Rust target triple (riscv64gc-unknown-linux-gnu)

The correct Rust target triple for riscv64 Linux is
riscv64gc-unknown-linux-gnu, not riscv64-unknown-linux-gnu.
The gc suffix denotes the G (general-purpose) + C (compressed)
ISA extensions that the standard Linux ABI requires.

Also fix the matching CARGO_TARGET_ env var name in the Dockerfile
so Cargo picks up the cross linker for the correct target.

Signed-off-by: Bruno Verachten <gounthar@gmail.com>

.github/workflows/main.yml

@@ -30,7 +30,7 @@ jobs:
 
           - artifact: riscv64-linux
             os: ubuntu-24.04
-            rust_target: riscv64-unknown-linux-gnu
+            rust_target: riscv64gc-unknown-linux-gnu
             cross_cmake_args: >-
               -DCMAKE_SYSTEM_NAME=Linux
               -DCMAKE_SYSTEM_PROCESSOR=riscv64

ci/docker/Dockerfile.riscv64-linux

@@ -23,7 +23,7 @@ ENV CC=riscv64-linux-gnu-gcc
 ENV CXX=riscv64-linux-gnu-g++
 
 # Cargo needs an explicit linker when cross-compiling for riscv64.
-ENV CARGO_TARGET_RISCV64_UNKNOWN_LINUX_GNU_LINKER=riscv64-linux-gnu-gcc
+ENV CARGO_TARGET_RISCV64GC_UNKNOWN_LINUX_GNU_LINKER=riscv64-linux-gnu-gcc
 
 # Tell programs to cache in a location that both isn't a `--volume` mounted root
 # and isn't `/root` in the container as that won't be writable during the build.