v.io/jiri: add policy driven flags for merging environment variables.
v.io/jiri: add policy driven flags for merging environment variables.

- Also tidy up v23-profile env, list etc.

MultiPart: 4/11

Change-Id: I6f8044e44c1719efad858a34f9114f3254ca6929

vanadium/browser,vanadium/browser,vanadium/browser,vanadium/browser