Preserve caller-provided RUSTDOCFLAGS in cargo-zerocopy

[fallintoplace]

This fixes a mismatch in cargo-zerocopy's flag forwarding for docs builds.

cargo-zerocopy previously read RUSTFLAGS and then reused that value for both RUSTFLAGS and RUSTDOCFLAGS. In CI, the docs job appends docs.rs-specific rustdoc args and --document-hidden-items to RUSTDOCFLAGS, so those rustdoc-only flags were getting dropped before cargo doc ran.

What changed:

• read both RUSTFLAGS and RUSTDOCFLAGS independently
• keep the wrapper's internal cfg flags on both channels
• add unit tests covering appended caller flags, empty caller flags, and distinct rustdoc flags

Developer impact:

• the docs CI step now exercises the rustdoc configuration it says it is exercising
• caller-provided rustdoc flags are no longer silently replaced by caller-provided rust flags

Validation:

• cargo fmt -p cargo-zerocopy --manifest-path /Users/hoangvu/Code/OSS/zerocopy/tools/Cargo.toml --check
• cargo test -p cargo-zerocopy --manifest-path /Users/hoangvu/Code/OSS/zerocopy/tools/Cargo.toml

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.85%. Comparing base (ed09d09) to head (1cfb77c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3476   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files          20       20           
  Lines        6093     6093           
=======================================
  Hits         5597     5597           
  Misses        496      496           

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