Bump thirdparty/SingleApplication from `494772e` to `a8da87d` #93

Job	Run time
build (macos-latest, mac, 5.15.2)	2m 41s
build (ubuntu-latest, linux, 6.2.*, qtmultimedia)	1m 32s
build (windows-latest, windows, 5.12.12, 73)	2m 32s
build (ubuntu-latest, linux, 5.12.12)	1m 36s
	8m 21s

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