shared: Fix recovery after an interrupted build

When build process is interrupted (like when the buildout process terminated by SIGKILL), building with shared leave the system in a state that looks like installation succeeded, so next execution will assume the part was properly installed.

The behavior was something like this:

if isSharedPartSignatureOK():
  useThisAlreadyInstalledPart()
else:
  writeSignatureInSharedPart()
  try:
    build()
  except:
    removeSharedPart()
    raise

and when program is terminated during build step, the signature is there.

The fix is to write signature later, algorithm becomes:

if isSharedPartSignatureOK():
  useThisAlreadyInstalledPart()
else:
  try:
    build()
  except:
    removeSharedPart()
    raise
  writeSignatureInSharedPart()