Fix ./pants2 overriding preset interpreter constraints (#7334)
Fix ./pants2 overriding preset interpreter constraints (#7334)

### Problem
`./pants2` would always override `PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS`. This is not always desired, e.g. when we want to specify you must use Python 2.7.15 instead of 2.7.13.

This fixes an issue introduced by https://github.com/pantsbuild/pants/pull/7257.

### Solution
Allow the env var to be pre-set, else default to allowing Py2.7 or 3.6+.
pantsbuild/pants,twitter/pants,twitter/pants,twitter/pants,twitter/pants,jsirois/pants,benjyw/pants,benjyw/pants,twitter/pants,jsirois/pants,wisechengyi/pants,twitter/pants,benjyw/pants,wisechengyi/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,twitter/pants,tdyas/pants,tdyas/pants,wisechengyi/pants,tdyas/pants,wisechengyi/pants,tdyas/pants,tdyas/pants,tdyas/pants,twitter/pants,wisechengyi/pants,pantsbuild/pants,tdyas/pants,pantsbuild/pants,benjyw/pants,jsirois/pants,twitter/pants,tdyas/pants,benjyw/pants,wisechengyi/pants,wisechengyi/pants,wisechengyi/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants,wisechengyi/pants,tdyas/pants