USe cwd() not $ENV{PWD}

One liner to get rid of warn output shown below, which occurs because $ENV{PWD} is not set. $ENV{PWD} appears to be erroneous in any case, as it does not update with chdir() which has been called earlier at this point.

     sudo rsnapshot -c rsnapshot.conf sync
     Use of uninitialized value $linux_lvm_oldpwd in concatenation (.) or string at /usr/bin/rsnapshot line   3720.
    Use of uninitialized value $linux_lvm_oldpwd in chdir at /usr/bin/rsnapshot line 3722.
    Use of chdir('') or chdir(undef) as chdir() is deprecated at /usr/bin/rsnapshot line 3722.
    Logical volume "backups-rsnapshot" successfully removed

Test pass. Note this is a recommit of an older commit I did to master.