* Cirrus-CI: update to FreeBSD 13.3
* proc,go.mod: update x/sys remove KeepAlive calls
Update version of golang.org/x/sys to the latest version and remove
calls to runtime.KeepAlive calls that were added to the FreeBSD backend
to work around an issue in the version of x/sys that we were using.
Use godeps to vendor dependencies and the normal go toolchain, with
GO15VENDOREXPERIMENT=1 set to properly vendor dependencies for reproducible
builds.
