build-rvv-env.sh script issues

[effrey-liu]

The build-rvv-env.sh script In buddy-mlir/thirdparty directory has some problems.
First of all, in riscv-gnu-toolchain repo, there is no more rvv-next branch, which should be deleted.
Secondly, when we pull from large git repos, it alaways happens pull failure and takes a lot of time in pull process. For user-friendly reasons, I think it's better to run git clone or git submodule update commands in "while true" block. Like below:

#!/bin/bash
while true; do
    git submodule update --init --recursive
    if [ $? -eq 0 ]; then
        echo "Submodule update completed successfully."
        break
    else
        echo "Submodule update failed. Retrying in 5 seconds..."
        sleep 5
    fi
done

Thirdly, in "Build cross MLIR" and "Build cross Buddy-MLIR" process, leaving out the cd .. command.