Enable unified env by default (#650)

## Summary
I guess it _actually_ was ready right on time on 2/16 :). (but not for a
2/16 release)

## How was it tested?
testscript tests and using it while developing

Author: ipince

internal/boxcli/featureflag/unified_env.go

@@ -7,4 +7,4 @@ package featureflag
 // in impl.computeNixEnv().
 // The feature is called UnifiedEnv because we use the exact same environment for
 // both devbox shell and devbox run.
-var UnifiedEnv = disabled("UNIFIED_ENV")
+var UnifiedEnv = enabled("UNIFIED_ENV")