docker: fall back to a known bpftool version
docker: fall back to a known bpftool version

The `linux-tools-$(uname -r)` construct that 1f0ff4c6 added
only works if the host kernel is one of Ubuntu's.

If the install fails, fall back to a known package version and
replace the /sbin/bpftool script with a direct symlink

Fixes: 1f0ff4c6 Dockerfile.focal: install linux-tools

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace