Fix "^" in --match flag for nanobench SKPs

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1706923002

Review URL: https://codereview.chromium.org/1706923002

bench/nanobench.cpp

@@ -609,7 +609,7 @@ public:
     static bool ReadPicture(const char* path, SkAutoTUnref<SkPicture>* pic) {
         // Not strictly necessary, as it will be checked again later,
         // but helps to avoid a lot of pointless work if we're going to skip it.
-        if (SkCommandLineFlags::ShouldSkip(FLAGS_match, path)) {
+        if (SkCommandLineFlags::ShouldSkip(FLAGS_match, SkOSPath::Basename(path).c_str())) {
             return false;
         }