[CT] Trim win path prefixes from filename as well

Bug: skia:10527
Change-Id: I973936427ecdafe5e2608dfdc61f99c808fad277
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/305530
Reviewed-by: Ravi Mistry <rmistry@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>
diff --git a/go/isolate/isolate.go b/go/isolate/isolate.go
index a7d08e6..b2ccbc1 100644
--- a/go/isolate/isolate.go
+++ b/go/isolate/isolate.go
@@ -406,6 +406,7 @@
 		if runtime.GOOS == "windows" {
 			// Win path prefixes seem to confuse isolate server.
 			dirname = strings.TrimPrefix(dirname, `c:`)
+			filename = strings.TrimPrefix(filename, `c:`)
 		}
 		cmd = append(cmd, "--files", fmt.Sprintf("%s:%s", dirname, filename))
 	}