CONTRIBUTING.md coverage broken by removal of -Zprofile flag

[karanabe]

Problem

The Rust compiler has removed support for the -Zprofile flag (see rust-lang/rust#131829), so the current coverage instructions in CONTRIBUTING.md no longer work.

Proposed Solution

Update the documentation to use the new LLVM-based instrumentation flags (-Cinstrument-coverage and LLVM_PROFILE_FILE) as shown in the current diff.