On some platforms, e.g. Linux Clang 22.1.8 / glibc 2.43, strchr()
now implements the C23 behaviour where passing a const pointer to
strchr() also returns a const pointer. This breaks xinstall during
the bootstrap build, since it assumes the return value is always
a mutable pointer.

As the returned pointer is never used to modify the value, fix this
by making the temporary variable const.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation