testing/oauth2-proxy: enable cgo to fix rv64
For riscv64, go does not support internal linking with -buildmode=pie
.
Before go1.22, it was silently accepted, but since go1.22, it's failing
to build.
Unless there is a good reason, we prefer to build with CGO, so remove the hardcoded flag in the Makefile.
Edited by Kevin Daudt