Add read-only permission to workflows #57

Job	Run time
--enable-hpcombi --disable-eigen (macOS-latest)	5m 31s
--disable-hpcombi --disable-eigen (macOS-latest)	5m 47s
--enable-hpcombi --disable-eigen (ubuntu-latest)	8m 41s
--disable-hpcombi --disable-eigen (ubuntu-latest)	8m 16s
--disable-hpcombi --enable-eigen (ubuntu-latest)	9m 4s
--disable-hpcombi --enable-eigen (macOS-latest)	8m 29s
	45m 48s

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