Environments: fix bug for install of external packages (#10437)

"spack install" will install all packages added to the current
environment. When this included external packages, the environment
update would fail because it would attempt to copy log files that
were only generated if Spack handled the install itself. This skips
that step for external packages.

lib/spack/spack/environment.py

@@ -663,6 +663,7 @@ def install_all(self, args=None):
             with fs.working_dir(self.path):
                 spec.package.do_install(**kwargs)
 
+            if not spec.external:
                 # Link the resulting log file into logs dir
                 build_log_link = os.path.join(
                     log_path, '%s-%s.log' % (spec.name, spec.dag_hash(7)))