feat(install): plugins support APOLLO_ROVER_DOWNLOAD_HOST

Co-authored-by: Avery Harnish <[email protected]>

docs/source/getting-started.md

@@ -49,6 +49,7 @@ Rover is distributed on npm for integration with your JavaScript projects.
 Internally, the `npm` installer downloads router binaries from `https://rover.apollo.dev`. If this URL is unavailable, for example, in a private network, you can point the `npm` installer at another URL in one of two ways:
 
 - by setting the `APOLLO_ROVER_DOWNLOAD_HOST` environment variable
+ - *note*: This value must be set if you also want the rover plugins to be downloaded from this host
 - by adding the following to your global or local `.npmrc`:
 
 ```ini

src/command/install/plugin.rs

@@ -92,12 +92,17 @@ impl Plugin {
 
     pub fn get_tarball_url(&self) -> RoverResult<String> {
         Ok(format!(
-            "https://rover.apollo.dev/tar/{name}/{target_arch}/{version}",
+            "{host}/tar/{name}/{target_arch}/{version}",
+            host = self.get_host(),
             name = self.get_name(),
             target_arch = self.get_target_arch()?,
             version = self.get_tarball_version()
         ))
     }
+
+    fn get_host($self) -> String {
+        std::env::var("APOLLO_ROVER_DOWNLOAD_HOST").unwrap_or_else(|| "https://rover.apollo.dev".to_string())
+    }
 }
 
 impl FromStr for Plugin {