Use --no-depexts instead of --assume-depexts

Means that packages which are unavailable can still be installed, as in
opam 2.0.

master_changes.md

@@ -10,7 +10,7 @@ New option/command/subcommand are prefixed with ◈.
   *
   * Fix typo in error message for opam var [#4786 @kit-ty-kate - fix #4785]
   * Add cli 2.2 handling [#4853 @rjbou]
-  * --assume-depext is the default in CLI 2.0 mode [#4908 @dra27]
+  * --no-depexts is the default in CLI 2.0 mode [#4908 @dra27]
 
 ## Plugins
   *

src/client/opamArg.ml

@@ -624,7 +624,7 @@ let apply_build_options cli b =
        OpamPackage.Name.Set.of_list)
     ?unlock_base:(flag b.unlock_base)
     ?locked:(if b.locked then Some (Some b.lock_suffix) else None)
-    ?no_depexts:(flag b.no_depexts)
+    ?no_depexts:(flag b.no_depexts || OpamCLIVersion.Op.(cli @= cli2_0))
     ();
   OpamClientConfig.update
     ?keep_build_dir:(flag b.keep_build_dir)
@@ -633,7 +633,7 @@ let apply_build_options cli b =
     ?show:(flag b.show)
     ?fake:(flag b.fake)
     ?skip_dev_update:(flag b.skip_update)
-    ?assume_depexts:(flag (b.assume_depexts || b.no_depexts || OpamCLIVersion.Op.(cli @= cli2_0)))
+    ?assume_depexts:(flag (b.assume_depexts || b.no_depexts))
     ~scrubbed_environment_variables
     ()