feat: match bash installer env, attestation, and path handling

[stevencartavia]

Brings the Rust port in line with the legacy foundryup bash script:

• Reads FOUNDRYUP_* env vars as argument defaults, treating empty ones as unset
• Uses the release tag (not the asset version) for the attestation cache check, so nightlies don't needlessly re-download
• --use now respects --repo
• Resolves a local --path to an absolute path before symlinking
• Removes dangling symlinks and replaces existing build artifacts (rm -f / mv -f behavior)
• Only treats nightly-<sha> as the nightly channel, and treats empty RUSTFLAGS as unset
• Accepts mac* / win* / mingw* platform names

Adds tests for env handling, version normalization, and platform/arch parsing.

Part of OSS-334