fix: use native exec proc to avoid sanitization of the toolkit one

[crazy-max]

just testing with child_process as the one from GitHub core toolkit sanitizes the command being passed which leads to issues like #40.

works fine: https://github.com/dagger/dagger-for-github/runs/6104146243?check_suite_focus=true#step:3:32

11:48PM INF actions.test._exec | computing
11:48PM INF actions.test._exec | completed    duration=100ms
11:48PM INF actions.test._exec | #5 0.076 hello GitHub Actions

but not quite happy about it as streamed output leaks in collapsible groups: https://github.com/dagger/dagger-for-github/runs/6104146243?check_suite_focus=true#step:3:18

Signed-off-by: CrazyMax [email protected]