Did you try the settings in RStudio Option that @heramb adviced ?
The setting through windows pane seems correct. Note that it required a reboot of the computer to take effect - I think the variable are loaded into the session when started.
It is like the PATH variable is not taken into account by RStudio. Which version of Rstudio is this ?
As a workaround to get command line in RStudio, you could be able to open a shell from the git pane (not the Rstudio Terminal).
In the Terminal Global option, which terminal did you select ?