environment.py: only install root specs explicitly (#31645)

lib/spack/spack/cmd/install.py

@@ -131,8 +131,9 @@ def setup_parser(subparser):
         help='(with environment) only install already concretized specs')
     subparser.add_argument(
         '--no-add', action='store_true', default=False,
-        help="""(with environment) only install specs provided as argument
-if they are already in the concretized environment""")
+        help="""(with environment) partially install an environment, limiting
+to concrete specs in the environment matching the arguments.
+Non-roots remain installed implicitly.""")
     subparser.add_argument(
         '-f', '--file', action='append', default=[],
         dest='specfiles', metavar='SPEC_YAML_FILE',

lib/spack/spack/environment/environment.py

@@ -1643,7 +1643,9 @@ def install_specs(self, specs=None, **install_args):
 
         installs = []
         for spec in specs_to_install:
-            installs.append((spec.package, install_args))
+            pkg_install_args = install_args.copy()
+            pkg_install_args['explicit'] = spec in self.roots()
+            installs.append((spec.package, pkg_install_args))
 
         try:
             builder = PackageInstaller(installs)