Allow manual runs of main CI workflow #326

Job	Run time
Windows (x86_64) release (crossbuild from Ubuntu x86_64, MinGW-w64)	2m 1s
Ubuntu (x86_64) debug	1m 3s
macOS (x86_64) release	1m 10s
Ubuntu (x86_64) release	1m 47s
	6m 1s

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