Allows port to be specified as an environment variable #1341

Job	Run time
R-CMD-check / setup	56s
routine / routine	3m 28s
website / pkgdown	2m 49s
R-CMD-check / macOS-latest (4.4.2)	3m 9s
R-CMD-check / windows-latest (4.4.2)	3m 9s
R-CMD-check / ubuntu-latest (devel)	3m 8s
R-CMD-check / ubuntu-latest (4.4.2)	2m 31s
R-CMD-check / ubuntu-latest (4.3.3)	2m 31s
R-CMD-check / ubuntu-latest (4.2.3)	3m 40s
R-CMD-check / ubuntu-latest (4.1.3)	3m 4s
R-CMD-check / ubuntu-latest (4.0.5)	2m 53s
	31m 18s

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