R is not correctly configured for the proxy on Linux

[TonyWildish-BH]

Describe the bug
R and RStudio rely on /etc/R/* for configuration. The /etc/R/Rprofile file is set during system boot to point to Nexus. This code has a bug, so R still tries to use the official CRAN repositories by default. The result is that R cannot install packages correctly.

Steps to reproduce

1. Start a Linux VM
2. Start R or RStudio
3. Run install.packages( "tidyverse" )

The fix is straightforward, there are too many double-quotes in templates/workspace_services/guacamole/user_resources/guacamole-azure-linuxvm/terraform/vm_config.sh, at line 160.

echo -e "local({\n    r <- getOption(\"repos\")\n    r[\"Nexus\"] <- \"""${NEXUS_PROXY_URL}\"/repository/r-proxy/\"\n    options(repos = r)\n})" | tee /etc/R/Rprofile.site

needs to be replaced with

echo -e "local({\n    r <- getOption(\"repos\")\n    r[\"Nexus\"] <- \"${NEXUS_PROXY_URL}/repository/r-proxy/\"\n    options(repos = r)\n})" | tee /etc/R/Rprofile.site

that's one \" removed at the end of the proxy environment variable and a pair of extra double-quotes removed from the beginning of it.

This bug exists in the current head of the repo.