chore: fix spurious generate failures

```
fork/exec /tmp/go-build720301329/b001/exe/gen: no such file or directory
pkg/flannel/flannel.go:8: running "go": exit status 1
```

I believe the root cause might be cache miss, that might cause
`generate` target to be called from `WITH_RACE=true` target, enabling
`CGO_ENABLED=1`, so it is random to reproduce (depends on buildkit cache
behavior).

Signed-off-by: Andrey Smirnov <[email protected]>

pkg/flannel/flannel.go

@@ -5,4 +5,4 @@
 // Package flannel provides flannel default manifest.
 package flannel
 
-//go:generate go run ./gen.go
+//go:generate env CGO_ENABLED=0 go run ./gen.go