fix(migrations): Remove unexpected shell call

This patch removes the call of `/usr/bin/env` when calling the migration
script in favour of using the processes own nodejs invocation path.

This should drop the requirement for `/usr/bin/env` to exist on a
system/in a container that runs hedgedoc.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

public/docs/release-notes.md

@@ -2,6 +2,9 @@
 
 ## <i class="fa fa-tag"></i> 1.x.x <i class="fa fa-calendar-o"></i> UNRELEASED
 
+### Enhancements
+- Remove unexpected shell call during migrations
+
 This release drops support for Node.js 12, as it has reached end-of-life.