Add a new and optional pip_index setting when installing a tap/target
### Problem to solve
Some companies might block access to public PyPi and only allow internal/private PyPi. When this is the case, the current setup fails with error `<tap/target> could not be installed: failed to upgrade pip to the latest version.`
### Proposal
Add a new and optional `pip_index` setting when installing a tap/target
### What does success look like, and how can we measure that?
- A different PyPi can be used
- There is no error thrown out
### Regression test
(Ensure the feature doesn't cause any regressions)
- [ ] Write adequate test cases and submit test results
- [ ] Test results should be reviewed by a person from the team
### Links / references
_Please note that this was taken from GitLab, to be changed accordingly_
issue