Improve the caching of the downloaded sdks

[mit-mit]

From the review of https://github.com/dart-lang/setup-dart/pull/1 by @athomas:

Perhaps we should download this into ${RUNNER_TOOL_CACHE}/dart//

That would enable checking if the version is in the cache later. It would mean we'd have to resolve latest to a specific version though.

[kevmoo]

We could cache the etag, too – then use a HEAD request to compare?

Starting to get a bit complex for a bash script, though...

[kevmoo]

Ooo! See https://stackoverflow.com/a/12475760/39827

curl --dump-header header_dump www.google.no

 curl -I -v --header 'If-None-Match: "1d30-4c993ec28581d"' http://httpd.apache.org

[natebosch]

This could mitigate some of the flakiness we are seeing downloading the SDK.

[mit-mit]

Someone wants to take a stab at that in a PR? Then we can commit that, and try out the action pinned to that commit in some repo and see if that helps with flakiness?

[serverwentdown]

This has already been done. The current script uses RUNNER_TOOL_CACHE

[devoncarew]

From some investigation, I don't believe this action is using caching. We are writing the downloaded and uncompressed sdk into $RUNNER_TOOL_CACHE, but I don't believe that directory implies any automatic caching. I believe it's just the path to a directory with many pre-installed sdks for various languages (pre-installed for the specific image).