Extract the symlink-to-bpf-linker logic from integration-test to xtask
and use it in a new build script in integration-ebpf, causing ebpf
probes to be rebuilt when bpf-linker changes. Previously bpf-linker
changes would rebuild integration-test, but not integration-ebpf,
resulting in stale tests.
Note that this still doesn't address the possibility that a new
bpf-linker is added to the PATH ahead of the cached one. Solving this in
the general case would require rebuild-if-changed-env=PATH *and*
rebuild-if-changed={every-directory-in-PATH} which would likely mean far
too much cache invalidation.