Allow usage of pants2 script from other directories (#7409)
Allow usage of pants2 script from other directories (#7409)

### Problem

The `pants` script supports usage from other repositories in order to allow for running pants from sources in another buildroot. But the `pants2` script was not accounting for that usecase.

### Solution

Adapt `pants2` to locate itself in order to exec `pants`.
twitter/pants,twitter/pants,benjyw/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,tdyas/pants,benjyw/pants,twitter/pants,wisechengyi/pants,pantsbuild/pants,jsirois/pants,wisechengyi/pants,twitter/pants,tdyas/pants,twitter/pants,tdyas/pants,jsirois/pants,benjyw/pants,wisechengyi/pants,wisechengyi/pants,wisechengyi/pants,benjyw/pants,pantsbuild/pants,twitter/pants,tdyas/pants,twitter/pants,wisechengyi/pants,wisechengyi/pants,wisechengyi/pants,tdyas/pants,twitter/pants,jsirois/pants,pantsbuild/pants,benjyw/pants,tdyas/pants,tdyas/pants,wisechengyi/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,tdyas/pants,twitter/pants,tdyas/pants