Ignore PNG chunk CRCs in Rust PNG fuzzing builds

`rust/png/FFI.rs` already skips chunk CRC checks under
`cfg!(FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION)`, but nothing in Skia's
GN -> Bazel build set that cfg. As a result, fuzzers built with
`skia_build_fuzzers=true` rejected almost every mutated input with a CRC
`FormatError` before reaching the decoding logic.

Add a `//rust/png:fuzzing_build_mode` Bazel flag (mirroring
`//rust/png:for_android`) that passes the cfg to rustc, and set it from
GN when `skia_build_fuzzers` is true.

Bug: b/562799124
Change-Id: Iaa5e23396bd14eaeb59782af4e8297264a14b625
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1379816
Reviewed-by: Łukasz Anforowicz <lukasza@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Elliot Sisteron <elliotsisteron@google.com>