Allows port to be specified as an environment variable (#963) #1345

Job	Run time
R-CMD-check / setup	46s
routine / routine	3m 51s
website / pkgdown	3m 10s
R-CMD-check / macOS-latest (4.4.2)	2m 46s
R-CMD-check / windows-latest (4.4.2)	3m 23s
R-CMD-check / ubuntu-latest (devel)	2m 35s
R-CMD-check / ubuntu-latest (4.4.2)	2m 28s
R-CMD-check / ubuntu-latest (4.3.3)	2m 26s
R-CMD-check / ubuntu-latest (4.2.3)	2m 59s
R-CMD-check / ubuntu-latest (4.1.3)	3m 0s
R-CMD-check / ubuntu-latest (4.0.5)	2m 59s
	30m 23s

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