bug#42859: guix lint: permission denied for cve checker

[Jonathan Brielmaier]

When running guix lint with the cve checker I get the following error:

```
guix lint icecat --checkers=cve
fetching CVE database for 2020...view1 [cve]...
Backtrace:
          12 (primitive-load "/home/jonathan/.config/guix/current/bi?")
In guix/ui.scm:
  2046:12 11 (run-guix-command _ . _)
In srfi/srfi-1.scm:
    634:9 10 (for-each #<procedure 7fe83940afc0 at guix/scripts/lin?> ?)
In guix/scripts/lint.scm:
     60:4  9 (run-checkers #<package icecat@68.11.0-guix0-preview1 ?> ?)
In srfi/srfi-1.scm:
    634:9  8 (for-each #<procedure 7fe83094da20 at guix/scripts/lin?> ?)
In guix/scripts/lint.scm:
    69:21  7 (_ _)
In guix/lint.scm:
   1110:4  6 (check-vulnerabilities #<package icecat@68.11.0-guix0-?> ?)
   1102:9  5 (_ _)
In unknown file:
           4 (force #<promise #<procedure 7fe8387c8820 at guix/lint.?>)
In guix/lint.scm:
   1085:2  3 (_)
   1044:2  2 (call-with-networking-fail-safe _ _ _)
In ice-9/boot-9.scm:
  1736:10  1 (with-exception-handler _ _ #:unwind? _ # _)
  1669:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1669:16: In procedure raise-exception:
In procedure mkdir: Permission denied
```

I'm on a pretty new guix and I did observe this issue for quite a while:
```
guix describe
  guix 697e54b
    Repository-URL: https://git.savannah.gnu.org/git/guix.git
    Branch: master
    Commit: 697e54b2a16575afa9457456fa2a27b0e5895caa
```