Set `defaultNycOptions` via environment variables.

**Is your feature request related to a problem? Please describe.**
I am running my Cypress tests in parallel via GitHub Actions, on multiple VMs.
I want each tests coverage to be written to a different directory so that they may later be merged.
The `report-dir` for NYC therefore needs to differ for each parallelized test.
This repo only supports a static value for `report-dir` as defined as code in the NYC config..
I would like for the `report-dir` value to be variable and set at runtime by the GitHub Action.

**Describe the solution you'd like**
In `common-utils.js`, the `report-dir` property should be set to something like `process.env.CYPRESS_REPORT_DIR || './coverage'`, allowing me to set the `CYPRESS_REPORT_DIR` environment variable for each parallelized test in the action.

**Describe alternatives you've considered**
Adding a step to the action that writes a second NYC file (e.g. YAML) or overrides the JSON NYC config to set the `report-dir` manually, but I'm pretty opposed to having such manual script management in my actions when it can be abstracted here.

I've unblocked myself by converting my NYC config file from JSON to JS and referencing `process.env` there, but I don't like the repetition here: my NYC config references `NYC_REPORT_DIR` while my Cypress plugin file sets `NYC_REPORT_DIR`. I'm still having to manage an environment variable in code that could (should?) be abstracted away via native support.