Add opt-in internal vars to ndk-env

[StantonMatt]

Summary

• add cargo ndk-env --include-internal so cargo-ndk's internal linker wrapper variables stay hidden by default but can be exported when explicitly requested
• quote sh and PowerShell output directly instead of relying on debug formatting, so _CARGO_NDK_LDFLAGS keeps its internal separator when sourced
• document using the internal env in a subshell for direct cargo build

Fixes #185.

Testing

• Docker rust:1.86.0-bookworm: cargo fmt --all -- --check
• Docker rust:1.86.0-bookworm: cargo test --locked
• Docker rust:1.86.0-bookworm: cargo clippy --locked -- -D warnings
• Fake-NDK CLI smoke: default JSON hides internal vars; --include-internal JSON/sh/PowerShell includes them; sourced --include-internal --link-builtins output preserves _CARGO_NDK_LDFLAGS's separator