`tac` cannot handle seperators that are not UTF-8.

[collinfunk]

Using uutils built from commit f6d581f:

$ LC_ALL=en_US.iso8859-1 tac --separator=$(printf '\xe9') < /dev/null
error: invalid UTF-8 was detected in one or more arguments

For more information, try '--help'.

[martinkunkel2]

this is not specific to tac. Same for other tools as well, e.g.

$ LC_ALL=en_US.iso8859-1 ./target/debug/coreutils more --pattern=$(printf '\xe9')
error: invalid UTF-8 was detected in one or more arguments

For more information, try '--help'.

[ChrisDryden]

Added good first issue for this one if anyone is interested in taking it up: #9673 we have a bunch of example PR's of taking a option that's a string and converting it to OsString to make it non UTF-8 compatible.