Fix Server Actions compiler (#51318)
Make sure we are using `var` instead of `const` as we always put the new
appended statements to the end of the module body, but they can still be
referenced before in the HOC case in the runtime. This causes a runtime
error.
Tl;dr: `a = 1; var a` is fine, but `a; const a = ...` will result in a
compilation error.
Closes #49344.
