Post transaction hook errors when upgarding an alternative installation

[sidtronics]

Following errors occur when using --root or --sysroot flag:

[sid@arch]$ sudo pacman -Syu nano --root /mnt
:: Synchronizing package databases...
 core is up to date
 extra is up to date
 multilib is up to date
warning: nano-8.1-1 is up to date -- reinstalling
:: Starting full system upgrade...
resolving dependencies...
looking for conflicting packages...

Packages (1) nano-8.1-1

Total Installed Size:  2.56 MiB
Net Upgrade Size:      0.00 MiB

:: Proceed with installation? [Y/n]
(1/1) checking keys in keyring                     [######################] 100%
(1/1) checking package integrity                   [######################] 100%
(1/1) loading package files                         [######################] 100%
(1/1) checking for file conflicts                    [######################] 100%
(1/1) checking available disk space                [######################] 100%
:: Processing package changes...
(1/1) reinstalling nano                            [######################] 100%
:: Running post-transaction hooks...
(1/2) Arming ConditionNeedsUpdate...
(2/2) Checking which packages need to be rebuilt
join: /dev/fd/63: No such file or directory
comm: /dev/fd/62: No such file or directory
Error, do this: mount -t proc proc /proc
lscpu: failed to determine number of CPUs: /sys/devices/system/cpu/possible: No such file or directory
tee: /tmp/tmp.ECFhvycUPx/11398: No such file or directory
comm: /dev/fd/62: No such file or directory