Update environment variable GOOGLE_CLOUD_KEYFILE_JSON to use GOOGLE_APPLICATION_CREDENTIALS (#3522) (#2078)

Co-authored-by: Damian Myerscough <[email protected]>
Signed-off-by: Modular Magician <[email protected]>

Co-authored-by: Damian Myerscough <[email protected]>

Author: modular-magician

.changelog/3522.txt

@@ -0,0 +1,2 @@
+```release-note:none
+```

website/docs/guides/getting_started.html.markdown

@@ -158,10 +158,10 @@ file. Name it something you can remember, and store it somewhere secure on your
 machine.
 
 You supply the key to Terraform using the environment variable
-`GOOGLE_CLOUD_KEYFILE_JSON`, setting the value to the location of the file.
+`GOOGLE_APPLICATION_CREDENTIALS`, setting the value to the location of the file.
 
 ```bash
-export GOOGLE_CLOUD_KEYFILE_JSON={{path}}
+export GOOGLE_APPLICATION_CREDENTIALS={{path}}
 ```
 
 -> Remember to add this line to a startup file such as `bash_profile` or