feat: iOS release builds should run at full speed

[eseidel]

Right now shorebird release ios builds run about 10x slower than necessary.

This is because we compile with DART_SIMULATOR on always.

The fix is to make DART_SIMULATOR a runtime switch instead of a compile time switch. The simulator is only needed as part of running patched builds (and can be made much faster).

[eseidel]

We've decided to fix this by just fixing #674 instead. That will avoid us having two separate modes.