Run CI only on master in upstream but everywhere in forks (#369) #434

Job	Run time
mac (--features debugmozjs)	18m 36s
mac	9m 20s
windows (--features debugmozjs)	15m 0s
windows (--features debugmozjs, aarch64-uwp-windows-msvc)	14m 57s
windows (--features debugmozjs, x86_64-uwp-windows-msvc)	17m 8s
windows	12m 16s
windows (aarch64-uwp-windows-msvc)	11m 53s
windows (x86_64-uwp-windows-msvc)	13m 8s
linux (--features debugmozjs)	16m 24s
linux	8m 57s
Result	1s
	2h 17m 40s

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