go.mod,cmd/cigocacher: make PUTs async (#20578)

Pull in bradfitz/go-tool-cache#40, and configure PUTs to be async so we
never slow down the build or extend the build time for the sake of
writing to the remote cache.

Updates tailscale/corp#45334

Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>

flakehashes.json

@@ -4,7 +4,7 @@
     "sri": "sha256-TwIaPmGVHO9ZwdaO/jDStgWGQtKa4smS3er1i5aTNTw="
   },
   "vendor": {
-    "goModSum": "sha256-3KQ4C93byCm5ioPVBi+GC8mwonqhsdBdpBhUWTCZyoM=",
-    "sri": "sha256-7+zxTSp/uFcWHbZPqmffTzNUjCiwdKouAjqL1U5TV5k="
+    "goModSum": "sha256-AbvfXWjfcRZHLJOp2Vuslvie8ROmm2pGvhxYIs07aNY=",
+    "sri": "sha256-amKkUPszyhG4N5ZtrB01swBACYq76raSS+SQRneLmwc="
   }
 }