Don't force latest k6 when building extensions

an extension will always require k6 and a version of it. It makes little
sense to force newer version on users especially as we keep breaking
them and we need time to go fix them, but that can only happen after
release.

This still lets users specify the version (as latest for example) and
require the latest k6, it just isn't the default.

environment.go

@@ -118,9 +118,13 @@ func (b Builder) newEnvironment(ctx context.Context) (*environment, error) {
 	log.Println("[INFO] Pinning versions")
 	if b.K6Repo == "" {
 		// building with the default main repo
-		err = env.execGoModRequire(ctx, k6ModulePath, env.k6Version)
-		if err != nil {
-			return nil, err
+		if env.k6Version != "" {
+			// don't actually specify k6 version if it wasn't specified
+			// the extension(s) will require a version that they work with either way
+			err = env.execGoModRequire(ctx, k6ModulePath, env.k6Version)
+			if err != nil {
+				return nil, err
+			}
 		}
 	} else {
 		// building with a forked repo, so get the main one and replace it with