Propagate tags on prebuilt_runtimes to the filegroup (#6638)

These were already applied to the internal rule for the Clang-built
runtimes, but were then dropped from the filegroup which would often
negate their effect.

toolchain/driver/prebuilt_runtimes.bzl

@@ -174,4 +174,5 @@ def prebuilt_runtimes(name, target = None, tags = []):
         srcs = [
             ":" + name + "_clang",
         ],
+        tags = tags,
     )