feat(functions): use isolated-function #594

Job	Run time
matrix	8s
test (@browserless/benchmark)	42s
test (browserless)	1m 26s
test (@browserless/cli)	41s
test (@browserless/devices)	43s
test (@browserless/errors)	44s
test (@browserless/function)	1m 28s
test (@browserless/goto)	1m 46s
test (@browserless/lighthouse)	1m 13s
test (@browserless/pdf)	41s
test (@browserless/screencast)	46s
test (@browserless/screenshot)	55s
test (@browserless/test)	40s
finish	5s
	11m 58s

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