Skip migrations when reinstalling missing code

[PVince81]

Because the app was already installed, no migrations must be run at this
stage. Later on, the upgrade process might notice a version change and
run migrations anyway. This part ensures it doesn't happen twice during
the upgrade process.

• Requires Add flag to skip migrations on install core#27934

Tested as follows:

1. Install 9.1.5 enterprise
2. Enable user_ldap
3. Replace the source code with OC 10.0.1RC1 enterprise from tarball
4. Patch with Auto enable market app on update and improve app update logging core#27930
5. Patch with Add flag to skip migrations on install core#27934
6. Patch with this PR here
7. Edit config.php to point to the correct cert and staging marketplace (if applicable, need the user_ldap app to be there)
8. Run occ upgrade

Before the fixes: LDAP migrations run twice and getting TableAlreadyExistsException because it tries to create the same tables twice.
After the fix: upgrade went through! LDAP app still enabled.

[tomneedham]

👍