Allow precommand hook to export env vars in same shell

[shYkiSto]

Allow environment variables exported from the precommand hook to be available for subsequent commands. This is done by executing the precommand in the current shell context rather than a subshell ((eval "$PRECOMMAND")). This enables scenarios where the precommand can be used to prepare the environment for the main command execution.

// manifest.json
{
   "hooks" {
       "precommand": ". tools/jazelle/hooks/prepare_env.sh"
   }
}

// tools/jazelle/hooks/prepare_env.sh
#!/bin/bash
export SOME_CRITICAL_ENV_VAR="test_precommand_env"

% jz yarn node -p "process.env.SOME_CRITICAL_ENV_VAR"
test_precommand_env