performance: speed up spack find in environments

`Environment.added_specs()` has a loop around calls to
`Package.installed()`, which can result in repeated DB queries.  Optimize
this with a read transaction in `Environment`.

lib/spack/spack/environment.py

@@ -25,6 +25,7 @@
 import spack.repo
 import spack.schema.env
 import spack.spec
+import spack.store
 import spack.util.spack_json as sjson
 import spack.util.spack_yaml as syaml
 import spack.config
@@ -1235,6 +1236,9 @@ def added_specs(self):
         Yields the user spec for non-concretized specs, and the concrete
         spec for already concretized but not yet installed specs.
         """
+        # use a transaction to avoid overhead of repeated calls
+        # to `package.installed`
+        with spack.store.db.read_transaction():
             concretized = dict(self.concretized_specs())
             for spec in self.user_specs:
                 concrete = concretized.get(spec)