Broken `flutter` command, expecting writable stamp file

Not sure how I got here, but `flutter` in my workshop doesn't work no more:
```
$ flutter doctor
/var/lib/workshop/sdk/flutter/bin/internal/update_engine_version.sh: line 64: /var/lib/workshop/sdk/flutter/bin/cache/engine.stamp: Read-only file system
```
Flutter makes a lot of assumptions about its environment, one of which is that it's able to update itself… which doesn't play well with Workshop's SDK design…

Start with /var/lib/workshop/sdk/flutter/bin/internal/update_engine_version.sh and reproduce the failure with `flutter doctor`. Trace how the SDK handles engine.stamp in the read-only Workshop environment, then verify that `flutter doctor` completes without requiring a writable SDK cache.

Written by the indexing model from the issue text.