In runenv.t, don't automatically add -I../lib to the test program's arguments.

In runenv.t, don't automatically add -I../lib to the test program's arguments.

Instead, add -I../lib explicitly to the program's arguments when needed.
This makes the test pass with miniperl, and simplifies the later tests which
inspect the entirety of @INC.

miniperl will have been failing since 5e4c4c91bd52a48d, because it now loads
$INC[0]/buildcustomize.pl if present, resetting @INC, and always providing
-I../lib will cause it to find the buildcustomize.pl script.