CI: checkout v4, upload-artifact v4, action-gh-release v2, flatpak-builder v6, install-boost v2.4.5 #1419

Job	Run time
Build under Flatpak	4m 50s
Build on Windows (Release, COF=OFF)	3m 12s
Build on Linux (Release)	2m 37s
Build on Windows (Release, COF=ON)	6m 12s
Build on Linux (Debug)	1m 52s
Build on Windows (Debug, COF=OFF)	2m 50s
Build on Windows (Debug, COF=ON)	3m 27s
	25m 0s

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