Consider renaming PIP_RUN_MODE

[jaraco]

In #82, I'm considering renaming the variable name "mode". The name is too general - doesn't convey its specific purpose.

Here are some alternatives I'm considering:

• target: building on the name of the parameter used by pip
• install target: incorporating the name of the command and parameter used by pip
• env: inspired by the name of the "environment"
• environment: full spelling of above
• retention: captures the concept around persisting or not
• strategy suffix: any of the above with "strategy" appended

I'm liking PIP_RUN_ENVIRONMENT_STRATEGY and PIP_RUN_RETENTION_STRATEGY best.

[jaraco]

I'm planning to proceed with (PIPRUN) RETENTION_STRATEGY. It's a little verbose, but communicates the purpose best, IMO.