x/vuln/cmd/govulncheck: provide an option to scan multiple binaries in a directory

[hyangah]

Would be nice if it provides an option to scan multiple binaries easily.

I wanted to check all the binaries in my GOPATH/bin/* as I usually do with go version ..

I can see govulncheck . or govulncheck * can be tricky since
the command switches to the binary scan mode if the arg refers to an executable.
If that's complicated, providing a flag or subcommand to allow users to explicitly select the binary scanning mode is an option.