Reimplement +x execute bit on script

[dylan-smith]

We rolled this change back because it didn't work on MacOS ( #279 #297 ).

We should revisit this and see if we can implement it properly.

Would also probably be a good idea to update our e2e tests to invoke the script directly on linux/mac (instead of doing pwsh -File migrate.ps1) to get a bit of testing around this.

cc\ @tspascoal