fix: test failure on riscv64

[hack3ric]

Those tests' results are affected by unsupported platform error. Adding riscv64 to platforms fixes the issue on it.

Error log:

failures:

---- task::task_graph::test::test_multi_env_defaults_ambigu stdout ----
note: test did not panic as expected
---- task::task_environment::tests::test_find_task_explicit_defined stdout ----
thread 'task::task_environment::tests::test_find_task_explicit_defined' panicked at src/task/task_environment.rs:272:9:
assertion failed: matches!(result, Err(FindTaskError::AmbiguousTask(_)))
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- task::task_environment::tests::test_find_task_dual_defined stdout ----
thread 'task::task_environment::tests::test_find_task_dual_defined' panicked at src/task/task_environment.rs:246:9:
assertion failed: matches!(result, Err(FindTaskError::AmbiguousTask(_)))

---- task::task_environment::tests::test_find_ambiguous_task stdout ----
thread 'task::task_environment::tests::test_find_ambiguous_task' panicked at src/task/task_environment.rs:337:9:
assertion failed: matches!(result, Err(FindTaskError::AmbiguousTask(_)))

---- task::task_graph::test::test_custom_command stdout ----
thread 'task::task_graph::test::test_custom_command' panicked at src/task/task_graph.rs:322:10:
called `Result::unwrap()` on an `Err` value: UnsupportedPlatform(UnsupportedPlatform(UnsupportedPlatformError { environments_platforms: [Linux64, Win64, Osx64, OsxArm64], environment: Default, platform: LinuxRiscv64 }))


failures:
    task::task_environment::tests::test_find_ambiguous_task
    task::task_environment::tests::test_find_task_dual_defined
    task::task_environment::tests::test_find_task_explicit_defined
    task::task_graph::test::test_custom_command
    task::task_graph::test::test_multi_env_defaults_ambigu

test result: FAILED. 125 passed; 5 failed; 0 ignored; 0 measured; 0 filtered out; finished in 24.84s

[ruben-arts]

Cool wasn't aware we had riscv users! Thanks!