internal/task: check the error from Git.Clone in FakeCloudBuild.RunScript

This should fix the nil-panic reported in golang/go#63239.
(I suspect that it will expose some other failure mode, though.)

Fixes golang/go#63239.

Change-Id: I4f432c569d632863fc928c5fea5764b163979456
Reviewed-on: https://go-review.googlesource.com/c/build/+/530737
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>

Author: Bryan C. Mills

internal/task/fakes.go

@@ -798,6 +798,9 @@ func (cb *FakeCloudBuild) RunScript(ctx context.Context, script string, gerritPr
 			return CloudBuild{}, err
 		}
 		dir, err := (&Git{}).Clone(ctx, repo.dir.dir)
+		if err != nil {
+			return CloudBuild{}, err
+		}
 		defer dir.Close()
 		wd = dir.dir
 	} else {