ci: fix path issue for rmarkdown rendering

Refs #23
1 job for master in 1 minute and 12 seconds (queued for 3 seconds)
Status Name Job ID Coverage
failed deploy #168124


Name Stage Failure
deploy Test
> rmarkdown::render(file.path(irmara:::app_sys("app/www/documentation/documentation.Rmd")))
Error in abs_path(input) : The file '' does not exist.
Calls: <Anonymous> -> setwd -> dirname -> abs_path
In addition: There were 16 warnings (use warnings() to see them)

Execution halted
The command '/bin/sh -c R -e 'rmarkdown::render(file.path(irmara:::app_sys("app/www/documentation/documentation.Rmd")))'' returned a non-zero code: 1
Connection to closed.
Cleaning up file based variables
ERROR: Job failed: exit code 1