CI: matrix, cache, arm-runner

[llerafel]

Continuation of previous PR [#832]

Added:

• Introduced matrix build for amd64/arm64 with separate runners;
• Added cache (gha/dh?);
• Disable cron in forks (a.k.a. Playable Skyrim Together Build)

Scenario	amd/64 First	amd/64 Cache	arm/64 First	arm/64 Cache	Action Time	Action Time (Cache)	Delta Time (sec)	Delta Cache (sec)
now	11m 21.7s	-	1h 35m 56.6s	-	1h 49m 58s	-	*	*
matrix	10m 42.1s	9m 27.2s	1h 29m 34.2s	1h 25m 12.6s	1h 49m 35s	1h 36m 22s	−23s	−13m 36s
+another layer xmake config	10m 45.9s	2m 58.7s	1h 30m 39.9s	34m 4.1s	1h 46m 20s	0h 48m 45s	−3m 38s	−1h 1m 13s

* Local runs via act; arm64 was emulated with QEMU (no dedicated runner).

[Force67]

What's with the emoji?

[Force67]

What's with the emoji?