Fix a crash in sandbox.sh when the XDG cache directory does not exist

[kit-ty-kate]

Opam 2.1 has support for symlinked cache directory, however currently if the cache directory does not exist, the script will crash with no error message.

~/opam-repository$ ls ~/.cache
ls: cannot access '/home/opam/.cache': No such file or directory
~/opam-repository$ OPAM_SWITCH_PREFIX ~/.opam/opam-init/hooks/sandbox.sh build ls
bash: OPAM_SWITCH_PREFIX: command not found
~/opam-repository$ OPAM_SWITCH_PREFIX=blah ~/.opam/opam-init/hooks/sandbox.sh build ls
~/opam-repository$ echo $?
1
~/opam-repository$ sed -i 's/readlink -f/readlink -m/' ~/.opam/opam-init/hooks/sandbox.sh
~/opam-repository$ OPAM_SWITCH_PREFIX=blah ~/.opam/opam-init/hooks/sandbox.sh build ls
CHANGES.md  CONTRIBUTING.md  COPYING  README.md  packages  repo  version

[kit-ty-kate]

The script also missed some quotes

[rjbou]

Thanks!