tr: missing ambiguous octal escape warning

[andrewliebenow]

❯ coreutils printf '(1)' | coreutils tr -d '\501'
()%

❯ coreutils printf '(1)' | /usr/bin/tr -d '\501'
/usr/bin/tr: warning: the ambiguous octal escape \501 is being
        interpreted as the 2-byte sequence \050, 1
)%

Splitting this out from #6777 so the main issue can get fixed faster