CI: Run all GHA checks on backport branch
- [ ] closes #39696
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

cc @simonjayhawkins 

How this was tested

I made a fake 1.3.x branch which is actually just a copy of master.

I applied these changes, and GHA triggered on my fork. I also verifyed that Installing the ssh key and uploading docs were skipped.
See the run here.
https://github.com/lithomas1/pandas/runs/3081634451?check_suite_focus=true
