Auto updater disabler (#12660)

Supersedes https://github.com/zed-industries/zed/pull/12659
Fixes https://github.com/zed-industries/zed/issues/12588

One of Zed's core features is our collaboration software. As such, it is
important that we notify the user when their RPC protocol is out of
date, and how to update it. This PR adds a mechanism to replace the
existing auto updater with a message explaining how to update Zed for
this environment.

Release Notes:

- N/A

crates/cli/src/main.rs

@@ -314,7 +314,7 @@ mod flatpak {
         if let Some(flatpak_dir) = get_flatpak_dir() {
             let mut args = vec!["/usr/bin/flatpak-spawn".into(), "--host".into()];
             args.append(&mut get_xdg_env_args());
-            args.push("--env=ZED_IS_FLATPAK_INSTALL=1".into());
+            args.push("--env=ZED_UPDATE_EXPLANATION=Please use flatpak to update zed".into());
             args.push(
                 format!(
                     "--env={EXTRA_LIB_ENV_NAME}={}",
@@ -347,7 +347,7 @@ mod flatpak {
         {
             if args.zed.is_none() {
                 args.zed = Some("/app/libexec/zed-editor".into());
-                env::set_var("ZED_IS_FLATPAK_INSTALL", "1");
+                env::set_var("ZED_UPDATE_EXPLANATION", "Please use flatpak to update zed");
             }
         }
         args