Adds an opt-in `stamp` mode that caches the extracted `vendor/`
directory in addition to the Composer download cache. On a warm
hit, `composer install` is effectively a no-op and shaves 2–5
minutes off a full Magento install.
Composer's download cache contains platform-specific binaries and
extracted archives that aren't safe to share across operating
systems. Add the runner OS as a key segment so a Linux job won't
restore a macOS-built cache (or vice versa).
GrayBot
Damien Retzinger
kristof-ringleff
Vinai Kopp