It looks like something has changed with GOPATH support. It might be time to migrate to a vendor based module layout in the main build tree... This needs investigating.
The dev tree builds libcap again. I guess this was a transient breakage. At some point, I should still try to figure out how to use vendoring to build in-tree though.
Indeed, it looks like GOPATH/GO111MODULE will be dropped in go1.17: https://blog.golang.org/go116-module-changes
This is fixed in libcap-2.51 via this commit: https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=1b59ff98484b5dd18bd1a0f99a002d67b6d773cd