chore: fix setup script errors

[zkpepe]

made the setup script more reliable:

• MIRRORS_BASE now errors if missing.
• RUST_CONFIGURE_ARGS defaults to empty.
• mkdir -p ninja avoids “already exists” errors.
• curl -fLo follows redirects and fails on HTTP errors.

simple, clear, and it just works.

[rustbot]

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[marcoieni]

Suggested change

	# Check and set default values for variables
	: "${MIRRORS_BASE:?MIRRORS_BASE is not set}"
	: "${RUST_CONFIGURE_ARGS:=}"
	# Fail is variable is not set
	: "${MIRRORS_BASE:?MIRRORS_BASE is not set}"
	# Set the variable to an empty string if it's undefined
	: "${RUST_CONFIGURE_ARGS:=}"

[marcoieni]

Thanks, this looks good, I just left a small improvements for the comment.
After you make the change, please squash your commits and I will approve 🙏