…licitly

`semver_check` cannot handle Go release candidate versions like
`1.23rc2`, and so the if statement here would produce an error like:

```
plz-out/gen/go/build_defs/go.build_defs:48:26: error: failed to parse version: Invalid Semantic Version

    if version != "" and semver_check(version, ">= 1.20.0") and install_std is None:
```

This moves the check for `install_std` being explicitly set to before
the semver check, which allows skipping it.

…ion constraint