GitHub CI: replace cache key counters

GitHub actually uses a bit complicated pattern matching for the keys:
https://docs.github.com/en/actions/guides/caching-dependencies-to-speed-up-workflows#using-contexts-to-create-cache-keys

Namely if exact hit is not found, the key is used as a prefix which
could cause using 'ffmpeg-cache-2' for 'ffmpeg-cache'. In addition
steps.<id>.outputs.cache-hit is set to false if the match is not exact
but the cache is actually restored from inexact match.

+ use ${{ runner.os }} as part of the cache key (currently not needed
but we may use the same key for different platform caches)

.github/scripts/macOS/prepare_free_sdks.sh

@@ -0,0 +1,6 @@
+#!/bin/sh -eux
+
+mkdir -p /var/tmp/sdks-free
+cd /var/tmp/sdks-free
+curl -S -LO https://www.ximea.com/downloads/recent/XIMEA_OSX_SP.dmg
+