Spring Initalizr now bundles a wrapper script for the build system. While
that wrapper has the necessary execute flag in the zip archive, that flag
is lost as the zip abstraction does not honor those.

The init command now makes sure to restore the execute flag on `mvnw`
and `gradlew` if necessary.

Unfortunately, this can't be tested as the Windows build would fail to
assert that the executable flag has been propertly set.

Closes gh-4392