Cache is keyed on the ESP-IDF version used in CI, so there shouldn't be any
cache invalidation issues when ESP-IDF version changes.
Restoring from cache takes approx 15s, compared to 2-3m to perform these
steps (ESP-IDF tools install, ESP-IDF clone, ESP-IDF submodule clone) the
first time.
Cache size is approx 1.6GB, the git clone is tweaked as much as possible to
keep the size down.
Signed-off-by: Angus Gratton <angus@redyak.com.au>
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