Fix exit code for run command

[adambabik]

Consider the following Markdown file:

```sh { "name": "test-singleline" }
failhereplease && echo single
```

```sh { "name": "test-multiline" }
failhereplease
echo multi
```

When executing it with runme run, the exit code for both should not be zero. This PR makes sure this is the actual behaviour.

The problem was in how we collect the environment variable. The solution was to switch to a clean up function instead of doing it inline, which could overwrite the original exit status.

Another problem, which is not covered in this PR, is to surface the actual shell exit code rather than any non-zero value convert to 1. This is due to an explicit conversion in:

runme/main.go

Lines 22 to 25 in 064e483

	if err := root.Execute(); err != nil {
	logf("could not execute command: %v\n", err)
	status = 1
	}

[sonarcloud]

Quality Gate passed

Issues
8 New issues

Measures
0 Security Hotspots
82.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[sourishkrout]

👍 LGTM. Let's create a follow-up issue for retaining the exit code as described.