Fix selecting and deleting user toolchains (#39068)

I was trying to use the new user toolchains but every time I clicked on
one I had added, it would delete it from the picker. Ironically, it
wouldn't delete it permanently when I tried to by clicking on the trash
can icon. Every time I reopened the workspace all user toolchains were
there.

Release Notes:

- Fixed selecting and deleting user toolchains.

crates/workspace/src/persistence.rs

@@ -997,6 +997,13 @@ impl WorkspaceDb {
                         }
                     }
                 }
+
+                conn.exec_bound(
+                    sql!(
+                        DELETE FROM user_toolchains WHERE workspace_id = ?1;
+                    )
+                )?(workspace.id).context("Clearing old user toolchains")?;
+
                 for (scope, toolchains) in workspace.user_toolchains {
                     for toolchain in toolchains {
                         let query = sql!(INSERT OR REPLACE INTO user_toolchains(remote_connection_id, workspace_id, worktree_id, relative_worktree_path, language_name, name, path, raw_json) VALUES (?1, ?2, ?3, ?4, ?5, ?6, ?7, ?8));