In EbuildFetcher._start(), don't touch the build dir when in prefetch mode.

In EbuildFetcher._start(), don't touch the build dir when in prefetch mode.
In this case, logging goes to emerge-fetch.log and the builddir should not be
touched since otherwise it could interfere with another instance of the same
cpv concurrently being built for a different $ROOT (currently, builds only
cooperate with prefetchers that are spawned for the same $ROOT). Thanks to
Daniel Robbins for reporting this issue. (trunk r12558)

svn path=/main/branches/2.1.6/; revision=12844