x/website: go.dev/dl was temporarily missing some port downloads

[prattmic]

For one hour after the 1.19.6 / 1.20.1 release, some ports, such as darwin-arm64, were missing from https://go.dev/dl/.

The likely cause is the metadata cache, which may have updated at just the wrong moment as port packages were published, causing it to only see some ports, and then cache that partial view for 1hr until expiration.

cc @golang/release

[dmitshur]

The /dl/upload endpoint has some logic to clear the cache at the end (here),
but it seems either that's not enough or something went wrong.