Fix chart linting in pipeline

Chart linting doesnt seem to work in the pipeline, although it works locally and in other repos...

It fails even when using an identical deployment strategy to the Benthos helm chart repo, which works. https://github.com/benthosdev/benthos-helm-chart/blob/main/.github/workflows/test.yaml

The only thing I can see which might be causing it to fail is this config in `chart-testing.yaml`
```yaml
chart-dirs:
- chart
```

This is the error that's returned when attempting to lint the chart:
```
Linting charts...
>>> helm version --template {{ .Version }}
>>> git rev-parse --is-inside-work-tree
Error: failed linting charts: failed identifying charts to process: must be in a git repository

------------------------------------------------------------------------------------------------------------------------
No chart changes detected.
------------------------------------------------------------------------------------------------------------------------
failed linting charts: failed identifying charts to process: must be in a git repository
make: *** [Makefile:6: chart-lint] Error 1
```