Remove OpenEXR install from github OSX workflow #237

Job	Run time
Build and test g++-9 -	5m 0s
Build and test g++-10 -	5m 6s
Build and test clang++-10 -	4m 46s
Build and test clang++-11 -	4m 51s
Build and test g++-9 - -DPBRT_DBG_LOGGING=True	3m 57s
Build and test g++-10 - -DPBRT_DBG_LOGGING=True	3m 41s
Build and test clang++-10 - -DPBRT_DBG_LOGGING=True	3m 44s
Build and test clang++-11 - -DPBRT_DBG_LOGGING=True	3m 27s
Build and test g++-9 - -DPBRT_FLOAT_AS_DOUBLE=True	4m 8s
Build and test g++-10 - -DPBRT_FLOAT_AS_DOUBLE=True	3m 38s
Build and test clang++-10 - -DPBRT_FLOAT_AS_DOUBLE=True	3m 48s
Build and test clang++-11 - -DPBRT_FLOAT_AS_DOUBLE=True	3m 51s
	49m 57s

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