The CGO_LDFLAGS we pass get embedded as the cgo_ldflag directive. Once
we pass -static, any cgo/net/os.user usage will lead to linking
statically. Avoid this.

Fixes: bazelbuild#3590

Control whether or not to link statically through go.mode.static
and not through the C/C++ compiler's ldflags.

This only matters when Go's linker decides to use external linker
for linking the final binary. This started to happen with Go 1.20
as the builder passes `-fdebug-prefix-map=` when building cgo code.

In return the Go compiler emits the preferlinkext hint.