infra: show RUSTFLAGS as well

[evverx]

It should make it easier to see where all the rustc flags
come from. RUSTFLAGS along with cargo fuzz build --verbose should
help to track down weird issues like #5865 (comment)

It's a follow-up to ecf3d38

[evverx]

Trying to figure out where I lost my backtraces, I noticed that in 4f7cf1b CUSTOM_LIBFUZZER_PATH and CUSTOM_LIBFUZZER_STD_CXX were introduced and it seems to be the only difference between the base-builder image and my local VM. I'm not sure I understand what those environment variables do. Could it be that they have something to do with #5865 (comment)?

Without those variables it's failing to compile in the base builder image:

^[[0m^[[0m^[[1m^[[32m   Compiling^[[0m toml-fuzz v0.0.0 (/src/toml-rs/fuzz)^M
^[[0m^[[0m^[[1m^[[32m     Running^[[0m `rustc --crate-name fuzz_from_to_string --edition=2018 fuzz_targets/fuzz_from_to_string.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type bin --emit=dep-info,link -C opt-level=3 -C embed-bitcode=no -C metadata=b333460274e8cb82 -C extra-filename=-b333460274e8cb82 --out-dir /src/toml-rs/fuzz/target/x86_64-unknown-linux-gnu/release/deps --target x86_64-unknown-linux-gnu -L dependency=/src/toml-rs/fuzz/target/x86_64-unknown-linux-gnu/release/deps -L dependency=/src/toml-rs/fuzz/target/release/deps --extern libfuzzer_sys=/src/toml-rs/fuzz/target/x86_64-unknown-linux-gnu/release/deps/liblibfuzzer_sys-03a713efd07706bd.rlib --extern toml=/src/toml-rs/fuzz/target/x86_64-unknown-linux-gnu/release/deps/libtoml-fcc8d3cc00d03c7f.rlib --cfg fuzzing -Cpasses=sancov -Cllvm-args=-sanitizer-coverage-level=4 -Cllvm-args=-sanitizer-coverage-trace-compares -Cllvm-args=-sanitizer-coverage-inline-8bit-counters -Cllvm-args=-sanitizer-coverage-pc-table -Clink-dead-code -Zsanitizer=address -Cllvm-args=-sanitizer-coverage-stack-depth -C codegen-units=1 --cfg fuzzing -Zsanitizer=address -Cdebuginfo=1 -Cforce-frame-pointers -L native=/src/toml-rs/fuzz/target/x86_64-unknown-linux-gnu/release/build/libfuzzer-sys-8fe07a06d6becf58/out`^M
^[[0m^[[1m^[[38;5;9merror^[[0m^[[0m^[[1m: linking with `cc` failed: exit status: 1^[[0m^M
"OUT" 885L, 235857B                                                                                                                                                                       1,1           Top
          /rust/registry/src/github.com-1ecc6299db9ec823/libfuzzer-sys-0.4.2/libfuzzer/FuzzerFork.cpp:85: undefined reference to `std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::~basic_string()'
          /rust/registry/src/github.com-1ecc6299db9ec823/libfuzzer-sys-0.4.2/libfuzzer/FuzzerFork.cpp:85: undefined reference to `std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::~basic_string()'
          /rust/registry/src/github.com-1ecc6299db9ec823/libfuzzer-sys-0.4.2/libfuzzer/FuzzerFork.cpp:85: undefined reference to `std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::~basic_string()'
          /src/toml-rs/fuzz/target/x86_64-unknown-linux-gnu/release/deps/liblibfuzzer_sys-03a713efd07706bd.rlib(FuzzerFork.o):/rust/registry/src/github.com-1ecc6299db9ec823/libfuzzer-sys-0.4.2/libfuzzer/FuzzerFork.cpp:85: more undefined references to `std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::~basic_string()' follow
          /src/toml-rs/fuzz/target/x86_64-unknown-linux-gnu/release/deps/liblibfuzzer_sys-03a713efd07706bd.rlib(FuzzerMutate.o): In function `std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::operator+=(char const*)':
          /usr/local/bin/../include/c++/v1/string:1009: undefined reference to `std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::append(char const*)'
          /src/toml-rs/fuzz/target/x86_64-unknown-linux-gnu/release/deps/liblibfuzzer_sys-03a713efd07706bd.rlib(FuzzerMutate.o): In function `std::__1::vector<fuzzer::MutationDispatcher::Mutator, fuzzer::fuzzer_allocator<fuzzer::MutationDispatcher::Mutator> >::__recommend(unsigned long) const':
          /usr/local/bin/../include/c++/v1/vector:1026: undefined reference to `std::__1::__vector_base_common<true>::__throw_length_error() const'
          /src/toml-rs/fuzz/target/x86_64-unknown-linux-gnu/release/deps/liblibfuzzer_sys-03a713efd07706bd.rlib(FuzzerMutate.o): In function `std::__1::vector<fuzzer::MutationDispatcher::Mutator, fuzzer::fuzzer_allocator<fuzzer::MutationDispatcher::Mutator> >::__vallocate(unsigned long)':

Apparently, cargo fuzz build in the base image builder is different from cargo fuzz build run locally. Looks like it has something to do with the rust toolchain/clang (maybe)

[jonathanmetzman]

LGTM