allow skipping mux permission check

When using WSL, we want to place the unix socket on NTFS and that
reports an insecure set of permissions which cause us to refuse
to start up.

As a bit of a gross hack, allow skipping that check by setting an
environmental variable:

```
WEZTERM_SKIP_MUX_SOCK_PERMISSIONS_CHECK= wezterm start --front-end muxserver
```

that works best in conjunction with this in the WSL `.wezterm.toml`
file: (swap `wez` with your username):

```
mux_server_unix_domain_socket_path = "/mnt/c/Users/wez/.local/share/wezterm/sock"
```

refs: #33

src/server/listener.rs

@@ -331,17 +331,19 @@ fn safely_create_sock_path(sock_path: &str) -> Result<UnixListener, Error> {
 
     #[cfg(unix)]
     {
-        // Let's be sure that the ownership looks sane
-        let meta = sock_dir.symlink_metadata()?;
-
-        let permissions = meta.permissions();
-        if (permissions.mode() & 0o22) != 0 {
-            bail!(
-                "The permissions for {} are insecure and currently
+        if !std::env::var_os("WEZTERM_SKIP_MUX_SOCK_PERMISSIONS_CHECK").is_some() {
+            // Let's be sure that the ownership looks sane
+            let meta = sock_dir.symlink_metadata()?;
+
+            let permissions = meta.permissions();
+            if (permissions.mode() & 0o22) != 0 {
+                bail!(
+                    "The permissions for {} are insecure and currently
                 allow other users to write to it (permissions={:?})",
-                sock_dir.display(),
-                permissions
-            );
+                    sock_dir.display(),
+                    permissions
+                );
+            }
         }
     }