fix build on windows

Author: chrisirhc

toolchain/defs.bzl

@@ -148,7 +148,7 @@ def _zig_repository_impl(repository_ctx):
     cache_prefix = repository_ctx.os.environ.get("HERMETIC_CC_TOOLCHAIN_CACHE_PREFIX", "")
     if cache_prefix == "":
         if os == "windows":
-            cache_prefix = "C:\\\\Temp\\\\zig-cache"
+            cache_prefix = "\\\\Temp\\\\zig-cache"
         elif os == "macos":
             cache_prefix = "/var/tmp/zig-cache"
         elif os == "linux":