cmd/go: checksum bypass for golang.org/fips140

Previously, a user operating inside of a malicious Go project that defines a bogus golang.org/fips140 and operates a malicious GOMODPROXY the user chooses to connect to can serve an arbitrary module in its place.

We now unpack the trusted ziphash for the bundled golang.org/fips140 module and construct its entry in the GOMODCACHE such that it can be verified by the toolchain.