[merlin, zk-sdk] Turn default-features for merlin crate to false

[samkim-crypto]

Problem

When building zk-sdk with cargo-build-sbf, I get the following error

error: target is not supported, for more information see: https://docs.rs/getrandom/#unsupported-targets
   --> src/lib.rs:290:9
    |
290 | /         compile_error!("target is not supported, for more information see: \
291 | |                         https://docs.rs/getrandom/#unsupported-targets");
    | |________________________________________________________________________^

   Compiling solana-sha256-hasher v2.2.0 (/Users/samkim/Projects/solana-program/agave/sdk/sha256-hasher)
error[E0433]: failed to resolve: use of undeclared crate or module `imp`
   --> src/lib.rs:341:9
    |
341 |         imp::getrandom_inner(dest)?;
    |         ^^^ use of undeclared crate or module `imp`

The default features in the merlin crate, which zk-sdk depends on uses rand_core.

[features]
default = ["std"]
nightly = []
debug-transcript = ["hex"]
std = ["rand_core/std", "byteorder/std"]

Summary of Changes

I set default-features = false for the merlin crate, which resolves the above error with cargo-build-sbf (thanks @joncinque!).

After I remove the default features, it complains that the transcript module inside zk-sdk is not used in target_os = "solana" builds. Not sure why it did not complain before, but I added excluded the transcript module from the target_os = "solana" target build.

Fixes #

[joncinque]

Looks good!