fix: cargo build registry-canister for wasm32 target

[ninegua]

The following command would fail to compile the registry canister if run from the /ic/rs/registery/canister sub-directory:

cargo build --profile canister-release --target wasm32-unknown-unknown --bin registry-canister

The fix is to make sure the feature getrandom/custom is enabled.

Note that the above command would succeed if run from the top-level directory, but would produce wrong wasm binary. This is because cargo would bring in global dependencies that enable both getrandom/custom and getrandom/js features, and the latter will lead to wasm binaries having unwanted imports (See #3309 for more details).

Since this problem does not affect bazel builds, this fix is only relevant to cargo.