Closes #683.

This PR fixes the installation code by polishing how rustup is installed:

• Making debian packages installation non-interactive by setting the corresponding environment variables.
• Fixing the rustup installation, which was broken in case rustup was not previously installed (which was never checked since in the CI we always installed previously rustup).

I confirm the whole flow has been successfully tested in a docker with ubuntu:latest by running the following commands:

export NONINTERACTIVE=1 && \
export DEBIAN_FRONTEND=noninteractive && \
apt-get update && \
apt-get install git curl gcc build-essential -y && \
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)" && \
eval "$(/home/linuxbrew/.linuxbrew/bin/brew shellenv)" \
cp /workdir/target/debug/pop /usr/local/bin &&
pop --version && \
unset DEBIAN_FRONTEND && \
pop install -y

In normal conditions we'd run brew install r0gue-io/pop-cli/pop (which yes, works on linux systems too) instead of copying it, but I already had the pop binary compiled for ubuntu in order to test the installation script with my local code.