Write an identical value to bin/cache/engine.stamp to prepare for migration (#164317)

Towards https://github.com/flutter/flutter/issues/164315.

This PR just writes `bin/cache/engine.stamp` identically to how
`bin/internal/engine.version` would otherwise be written, with a caveat
that _if_ `engine.version` is tracked, it is now _copied_ to
`bin/cache/engine.stamp`.

After this lands, I'll send PRs to update tooling that looks for
`engine.version` and give a heads up to the larger team (i.e. Dart HH
bot or whomever we will break by doing this).

.gitignore

@@ -1,9 +1,6 @@
 # Do not remove or rename entries in this file, only add new ones
 # See https://github.com/flutter/flutter/issues/128635 for more context.
 
-# This is dynamic in a monorepo
-bin/internal/engine.version
-
 # Miscellaneous
 *.class
 *.lock
@@ -33,6 +30,8 @@ bin/internal/engine.version
 /bin/cache/
 /bin/internal/bootstrap.bat
 /bin/internal/bootstrap.sh
+/bin/internal/engine.realm
+/bin/internal/engine.version
 /bin/mingit/
 /dev/benchmarks/mega_gallery/
 /dev/bots/.recipe_deps