Read env variable at runtime rather than compile time

Author: frazar

src/config.rs

@@ -199,7 +199,7 @@ impl Cfg {
             // If present, use the RUSTUP_OVERRIDE_UNIX_FALLBACK_SETTINGS environment
             // variable as settings path, or UNIX_FALLBACK_SETTINGS otherwise
             FallbackSettings::new(PathBuf::from(
-                option_env!("RUSTUP_OVERRIDE_UNIX_FALLBACK_SETTINGS")
+                process().var("RUSTUP_OVERRIDE_UNIX_FALLBACK_SETTINGS")
                     .unwrap_or(UNIX_FALLBACK_SETTINGS),
             ))?
         } else {