Adjust path to extracted grcov

Changed since mozilla-central revision 34ab60552eafeca2e94a759261f121d5844b334b, probably
due to https://bugzilla.mozilla.org/show_bug.cgi?id=1809351.

Author: marco-c

report/firefox_code_coverage/codecoverage.py

@@ -302,7 +302,7 @@ def download_grcov():
     os.remove(archive)
 
     # Get version from grcov binary
-    grcov = os.path.join(dest, "grcov")
+    grcov = os.path.join(dest, "grcov", "grcov")
     assert os.path.exists(grcov), "Missing grcov binary"
     version = subprocess.check_output([grcov, "--version"]).decode("utf-8")