list of available updates for globally installed packages

[Reza Alizadeh Majd]

Hi Guix, 

I asked before about a way to check if any update is available for
_Globally Installed Packages_ in `/run/current-system/profile` in this thread: 

https://lists.gnu.org/archive/html/help-guix/2019-10/msg00167.html

as suggested in above thread, I was pointing the target profile to achieve 
desired results: 

```
guix package -p /run/current-system/profile -n -u
```

but recently after updating the guix, I receive following error when I run 
above command. it seems that there is a permission issue occurs when guix tries 
to access to the system profile. 

```
panther@panther ~$ guix package -p /run/current-system/profile -n -u
Backtrace:
           8 (primitive-load "/home/panther/.config/guix/current/bin…")
In guix/ui.scm:
  1774:12  7 (run-guix-command _ . _)
In ice-9/boot-9.scm:
    829:9  6 (catch _ _ #<procedure 7f683b3981f8 at guix/ui.scm:647…> …)
    829:9  5 (catch _ _ #<procedure 7f683b398210 at guix/ui.scm:770…> …)
In guix/scripts/package.scm:
   945:10  4 (_)
In guix/status.scm:
    768:4  3 (call-with-status-report _ _)
In guix/scripts/package.scm:
   953:14  2 (_)
In guix/build/syscalls.scm:
  1124:14  1 (call-with-file-lock/no-wait _ #<procedure 7f682e1e9b9…> …)
In ice-9/boot-9.scm:
    777:6  0 (throw "open-file" "~A: ~S" ("Read-only file system" #) #)

ice-9/boot-9.scm:777:6: In procedure throw:
In procedure throw: Wrong type argument in position 1: open-file
```

could anyone help me on this, or alternatively suggest a working solution to 
check for list of available updates for packages that globally installed using 
`guix system reconfigure` ? 


--
Regards
Reza Alizadeh Majd
PantherX Team