Go toolchain vulnerabilities

Previously, a user operating inside a malicious Go project that defines a bogus golang.org/toolchain go.sum entry and operates a malicious GOMODPROXY could bypass the intended checksum. It is now ensured that golang.org/toolchain always goes to the network for the canonical checksum.

Previously, a user operating inside a malicious Go project that defines a bogus golang.org/fips140 and operates a malicious GOMODPROXY could serve an arbitrary module in its place. The trusted ziphash for the bundled golang.org/fips140 module is now unpacked and its entry in GOMODCACHE is constructed so it can be verified by the toolchain.

A malicious module proxy can exploit a flaw in the go command's validation of module checksums to bypass checksum database validation. This affects users using untrusted module proxies or checksum databases.