* fix: avoid downloading package when local cache exists
As mentioned in
https://github.com/Mozilla-Actions/sccache-action/issues/107,
`setup` function didn't use `find` function even though it uses
`cacheDir` to store downloaded files, which leads to redundant download.
It is not significant on GitHub-hosted runners because
`cacheDir` stores files in local file system, which is ephemeral.
However, it wastes time on self-hosted runners.
This commit adds a step to search local file system cache for sccache as
well as splitting download procedure to a dedicated function.
* fix: explicit comparison for boolean-like value
110416