crypto/x509: stop looking at first root store

[FiloSottile]

For some reason, when looking for a CA root store on UNIX we stop at the first good file, still look at directories, and not stop at the first good directory. On most systems the file, if it exists, is a bundling of the roots in the directory, and the directories are for different systems, not for combining.

We should probably make the function exit sooner.

[gopherbot]

Change https://golang.org/cl/234257 mentions this issue: crypto/x509: rework how system roots are loaded on unix systems

[FiloSottile]

Like #39540, let's give crypto/x509 a rest in Go 1.16.